<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:79092be0-d62d-47e4-8fea-b8f635cf5428,g:40e70129-93b7-4d85-8e08-29658eabcffd-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITY" xlink:type="simple" xlink:href="fsbc-20251231.xsd#CONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_244e4f11-71af-4c5e-bf06-53983468fe36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_190eb264-9432-4f14-8e61-e46ef4688629" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_244e4f11-71af-4c5e-bf06-53983468fe36" xlink:to="loc_us-gaap_StatementTable_190eb264-9432-4f14-8e61-e46ef4688629" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_02d2751a-5a81-4313-82bb-6bb73cb2d536" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_190eb264-9432-4f14-8e61-e46ef4688629" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_02d2751a-5a81-4313-82bb-6bb73cb2d536" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_02d2751a-5a81-4313-82bb-6bb73cb2d536_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_02d2751a-5a81-4313-82bb-6bb73cb2d536" xlink:to="loc_us-gaap_EquityComponentDomain_02d2751a-5a81-4313-82bb-6bb73cb2d536_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_37ceef02-5232-4a78-b48c-44922b76ae73" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_02d2751a-5a81-4313-82bb-6bb73cb2d536" xlink:to="loc_us-gaap_EquityComponentDomain_37ceef02-5232-4a78-b48c-44922b76ae73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_5c6ce68f-58c5-4eb7-8266-86f0ac5e9ac4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37ceef02-5232-4a78-b48c-44922b76ae73" xlink:to="loc_us-gaap_CommonStockMember_5c6ce68f-58c5-4eb7-8266-86f0ac5e9ac4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_0e460b6f-d973-4b0d-b46a-b4324db01a1b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37ceef02-5232-4a78-b48c-44922b76ae73" xlink:to="loc_us-gaap_RetainedEarningsMember_0e460b6f-d973-4b0d-b46a-b4324db01a1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_104995c7-0e69-46fd-9aa8-b749d2485b3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37ceef02-5232-4a78-b48c-44922b76ae73" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_104995c7-0e69-46fd-9aa8-b749d2485b3d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_fd299209-aa09-4ef4-b31b-35a550532897" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_190eb264-9432-4f14-8e61-e46ef4688629" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_fd299209-aa09-4ef4-b31b-35a550532897" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_fd299209-aa09-4ef4-b31b-35a550532897_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_fd299209-aa09-4ef4-b31b-35a550532897" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_fd299209-aa09-4ef4-b31b-35a550532897_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_28411b30-b8b4-4769-9494-27bca48fc1ee" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_fd299209-aa09-4ef4-b31b-35a550532897" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_28411b30-b8b4-4769-9494-27bca48fc1ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_a58e567f-bf43-40fc-8136-e31fbb6379ed" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_28411b30-b8b4-4769-9494-27bca48fc1ee" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_a58e567f-bf43-40fc-8136-e31fbb6379ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_244e4f11-71af-4c5e-bf06-53983468fe36" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_3de4a82e-ac43-4958-aaeb-0b1b1cc2d427" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_3de4a82e-ac43-4958-aaeb-0b1b1cc2d427" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_4665fdf3-ec96-4367-a78c-f6c0c403864b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_StockholdersEquity_4665fdf3-ec96-4367-a78c-f6c0c403864b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_3e596d90-1a2b-4a30-867c-85c2a0f6683a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_NetIncomeLoss_3e596d90-1a2b-4a30-867c-85c2a0f6683a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_5d14b028-3a00-40c5-be5f-2af9db7c0b11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_5d14b028-3a00-40c5-be5f-2af9db7c0b11" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_3aa7dbdb-5066-452f-ad73-f425c2e75dec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_3aa7dbdb-5066-452f-ad73-f425c2e75dec" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_ad633853-f388-4d4b-8fca-950d93fc69cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_ad633853-f388-4d4b-8fca-950d93fc69cb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_066b5464-1748-4085-8a63-001f282db1dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_066b5464-1748-4085-8a63-001f282db1dc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_4da317bd-7ca2-486a-802c-55e768cf09bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_4da317bd-7ca2-486a-802c-55e768cf09bc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_6c51a059-70f6-448c-a25e-e27d19912e32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_6c51a059-70f6-448c-a25e-e27d19912e32" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_5cba4647-fa02-4dd3-8202-85c0d10044dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9483c3b8-5e9d-4062-9185-d3f31c6d15ad" xlink:to="loc_us-gaap_DividendsCommonStock_5cba4647-fa02-4dd3-8202-85c0d10044dc" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_55b5d6dd-b322-4f98-99f3-b65cd53690a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_83ae54be-dc4b-4071-a237-ea3bdaa0b299" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="simple" xlink:href="fsbc-20251231.xsd#CONSOLIDATEDSTATEMENTSOFCASHFLOWS"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_06878de4-af6d-43cb-b5d5-d6d3f13f079f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:to="loc_us-gaap_StatementTable_06878de4-af6d-43cb-b5d5-d6d3f13f079f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_a9a4ae25-d463-4c8e-92f8-d42c288d39cb" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_06878de4-af6d-43cb-b5d5-d6d3f13f079f" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_a9a4ae25-d463-4c8e-92f8-d42c288d39cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_a9a4ae25-d463-4c8e-92f8-d42c288d39cb_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_a9a4ae25-d463-4c8e-92f8-d42c288d39cb" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_a9a4ae25-d463-4c8e-92f8-d42c288d39cb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e7eec95d-5610-4e37-8f15-51aea55c3842" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_a9a4ae25-d463-4c8e-92f8-d42c288d39cb" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e7eec95d-5610-4e37-8f15-51aea55c3842" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_a750d7d9-449b-464c-b97c-239e4ffb9778" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e7eec95d-5610-4e37-8f15-51aea55c3842" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_a750d7d9-449b-464c-b97c-239e4ffb9778" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2039c7f2-31c4-40e0-a571-7ddd6e257135" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_06878de4-af6d-43cb-b5d5-d6d3f13f079f" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2039c7f2-31c4-40e0-a571-7ddd6e257135" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_2039c7f2-31c4-40e0-a571-7ddd6e257135_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2039c7f2-31c4-40e0-a571-7ddd6e257135" xlink:to="loc_us-gaap_TypeOfAdoptionMember_2039c7f2-31c4-40e0-a571-7ddd6e257135_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_28e32719-9309-4452-86fc-2fd64f98b134" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2039c7f2-31c4-40e0-a571-7ddd6e257135" xlink:to="loc_us-gaap_TypeOfAdoptionMember_28e32719-9309-4452-86fc-2fd64f98b134" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdate201613Member_84e658a8-32a5-4542-a941-f19c6d35bb0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountingStandardsUpdate201613Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_28e32719-9309-4452-86fc-2fd64f98b134" xlink:to="loc_us-gaap_AccountingStandardsUpdate201613Member_84e658a8-32a5-4542-a941-f19c6d35bb0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019c56c9-3f91-716b-b803-46ad0cdc9bea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_06878de4-af6d-43cb-b5d5-d6d3f13f079f" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019c56c9-3f91-716b-b803-46ad0cdc9bea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019c56c9-3f91-716b-b803-46ad0cdc9bea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019c56c9-3f91-716b-b803-46ad0cdc9bea" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019c56c9-3f91-716b-b803-46ad0cdc9bea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019c56c9-3f91-7031-8ed7-8ed9d16e5d0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019c56c9-3f91-716b-b803-46ad0cdc9bea" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019c56c9-3f91-7031-8ed7-8ed9d16e5d0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_019c56c9-3f91-7cf2-bd0b-db8541f64116" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019c56c9-3f91-7031-8ed7-8ed9d16e5d0d" xlink:to="loc_stpr_CA_019c56c9-3f91-7cf2-bd0b-db8541f64116" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalTaxJurisdictionOtherMember_019c56c9-3f91-728b-a9ec-b84da611c0a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalTaxJurisdictionOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019c56c9-3f91-7031-8ed7-8ed9d16e5d0d" xlink:to="loc_us-gaap_StateAndLocalTaxJurisdictionOtherMember_019c56c9-3f91-728b-a9ec-b84da611c0a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_8ac72b8d-8b71-4806-b62f-8b112dc39dcb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_8ac72b8d-8b71-4806-b62f-8b112dc39dcb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_77f0ca8b-494b-4fcd-8999-abde8fdb25aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_8ac72b8d-8b71-4806-b62f-8b112dc39dcb" xlink:to="loc_us-gaap_NetIncomeLoss_77f0ca8b-494b-4fcd-8999-abde8fdb25aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_8ac72b8d-8b71-4806-b62f-8b112dc39dcb" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_b096ccc6-aadd-4392-bf93-547cc9145988" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_b096ccc6-aadd-4392-bf93-547cc9145988" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_047d496e-f715-4659-b66f-c480c27f980b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_DepreciationAndAmortization_047d496e-f715-4659-b66f-c480c27f980b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDeferredLoanOriginationFeesNet_ccaff186-bf8c-4f71-8a81-e693ac16794a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfDeferredLoanOriginationFeesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_AmortizationOfDeferredLoanOriginationFeesNet_ccaff186-bf8c-4f71-8a81-e693ac16794a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_832dda3e-87e7-48a7-8727-ab330a4d37b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_832dda3e-87e7-48a7-8727-ab330a4d37b7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_b829bf66-dd28-4625-899b-13fd16887983" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_b829bf66-dd28-4625-899b-13fd16887983" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization_cd53bae1-db23-4226-8912-604591764596" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization_cd53bae1-db23-4226-8912-604591764596" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_d3eee498-e4fc-4804-83e3-3ec2405152de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_ShareBasedCompensation_d3eee498-e4fc-4804-83e3-3ec2405152de" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankOwnedLifeInsuranceIncome_42977acb-c025-4365-9118-9933904917b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankOwnedLifeInsuranceIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_BankOwnedLifeInsuranceIncome_42977acb-c025-4365-9118-9933904917b2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_5ea269e4-f8ed-47ed-a6a2-8b8e10f8bc69" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_5ea269e4-f8ed-47ed-a6a2-8b8e10f8bc69" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PaymentsForTransferableTaxCredits_019ba0b2-fba0-73e3-9075-a9e77b51882b" xlink:href="fsbc-20251231.xsd#fsbc_PaymentsForTransferableTaxCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_fsbc_PaymentsForTransferableTaxCredits_019ba0b2-fba0-73e3-9075-a9e77b51882b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale_7e52ed55-5961-49b3-9883-660d358d93a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale_7e52ed55-5961-49b3-9883-660d358d93a7" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSalesOfLoansNet_f3713bc5-b8c1-45f5-a7cf-ab9cdd236871" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSalesOfLoansNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_GainLossOnSalesOfLoansNet_f3713bc5-b8c1-45f5-a7cf-ab9cdd236871" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfLoansHeldForSale_8315551d-05d3-4a2b-85da-728faf431cb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfLoansHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_ProceedsFromSaleOfLoansHeldForSale_8315551d-05d3-4a2b-85da-728faf431cb7" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss_5f2ca6b0-7fd7-4ed9-a7e7-5661b5e77082" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss_5f2ca6b0-7fd7-4ed9-a7e7-5661b5e77082" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal_b5f2d33f-a1c1-490d-9836-c2a83029099a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal_b5f2d33f-a1c1-490d-9836-c2a83029099a" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_2575918a-e155-4f73-aaff-2f62fc4a2d5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_2736f542-9e61-4385-a07c-d6bfb7f5f3ff" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_2575918a-e155-4f73-aaff-2f62fc4a2d5b" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet_b41a23c1-0593-4d32-bc3b-195c9c54af27" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_2575918a-e155-4f73-aaff-2f62fc4a2d5b" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet_b41a23c1-0593-4d32-bc3b-195c9c54af27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInInterestPayableNet_7b41186c-cf68-46c0-8267-3a5c40f7c9dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInInterestPayableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_2575918a-e155-4f73-aaff-2f62fc4a2d5b" xlink:to="loc_us-gaap_IncreaseDecreaseInInterestPayableNet_7b41186c-cf68-46c0-8267-3a5c40f7c9dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingLeaseLiability_70da3af0-1a91-42cf-b225-4baf9b5ec79e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_2575918a-e155-4f73-aaff-2f62fc4a2d5b" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingLeaseLiability_70da3af0-1a91-42cf-b225-4baf9b5ec79e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_6d1a286e-cd39-46e8-988a-0b05650a2006" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_8ac72b8d-8b71-4806-b62f-8b112dc39dcb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_6d1a286e-cd39-46e8-988a-0b05650a2006" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_172f3d16-1e1f-425e-86c6-626bb4c3644e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_172f3d16-1e1f-425e-86c6-626bb4c3644e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_79bc31e8-c674-4051-a2f0-140c5922e6b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_79bc31e8-c674-4051-a2f0-140c5922e6b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt_cca2bd1a-9f10-4ee4-b328-e9fe0622f486" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt_cca2bd1a-9f10-4ee4-b328-e9fe0622f486" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PaymentsForCapitalCallForEquityInvestment_b1998307-cd44-446d-b6eb-15ab49b73339" xlink:href="fsbc-20251231.xsd#fsbc_PaymentsForCapitalCallForEquityInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_fsbc_PaymentsForCapitalCallForEquityInvestment_b1998307-cd44-446d-b6eb-15ab49b73339" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfEquityMethodInvestments_f84df410-9a07-4740-a58c-746bc597ff54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_ProceedsFromSaleOfEquityMethodInvestments_f84df410-9a07-4740-a58c-746bc597ff54" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ProceedsFromPaymentsForCapitalCallLowIncomeHousingCredits_9d53c7ec-b6fa-47e0-8739-31ff9f8b998c" xlink:href="fsbc-20251231.xsd#fsbc_ProceedsFromPaymentsForCapitalCallLowIncomeHousingCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_fsbc_ProceedsFromPaymentsForCapitalCallLowIncomeHousingCredits_9d53c7ec-b6fa-47e0-8739-31ff9f8b998c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromPaymentsForInInterestBearingDepositsInBanks_193ee2d9-897a-484b-907c-f7e993ce0b60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromPaymentsForInInterestBearingDepositsInBanks"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_ProceedsFromPaymentsForInInterestBearingDepositsInBanks_193ee2d9-897a-484b-907c-f7e993ce0b60" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PaymentsForOriginationOfLoansNetOfProceedsFromRepayments_c1e85c4a-ebd7-4c35-8322-70651ce53dbc" xlink:href="fsbc-20251231.xsd#fsbc_PaymentsForOriginationOfLoansNetOfProceedsFromRepayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_fsbc_PaymentsForOriginationOfLoansNetOfProceedsFromRepayments_c1e85c4a-ebd7-4c35-8322-70651ce53dbc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_3181d47d-c8d0-4f7d-a222-ac9f0567af50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_3181d47d-c8d0-4f7d-a222-ac9f0567af50" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromFederalHomeLoanBankStock_eadd1acc-6cec-4d49-8984-202517e43545" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForProceedsFromFederalHomeLoanBankStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_PaymentsForProceedsFromFederalHomeLoanBankStock_eadd1acc-6cec-4d49-8984-202517e43545" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireLifeInsurancePolicies_1ed4bbc3-e470-4420-b2cd-24e2ef5410f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireLifeInsurancePolicies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_PaymentsToAcquireLifeInsurancePolicies_1ed4bbc3-e470-4420-b2cd-24e2ef5410f2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfOtherRealEstate_019ba0af-63eb-7346-b335-65cb82be1c25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfOtherRealEstate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_ProceedsFromSaleOfOtherRealEstate_019ba0af-63eb-7346-b335-65cb82be1c25" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_b2e987b9-ed7e-4a6c-9835-885a91f36462" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_c00596ce-f2a3-4333-b543-cfb132667b19" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_b2e987b9-ed7e-4a6c-9835-885a91f36462" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeposits_9f4a44bf-45fa-4ee8-a1cd-7beebb6f21f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_IncreaseDecreaseInDeposits_9f4a44bf-45fa-4ee8-a1cd-7beebb6f21f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_d8d97e85-1b90-473e-bea9-b87f05b2e196" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_d8d97e85-1b90-473e-bea9-b87f05b2e196" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromRepaymentsOfShortTermDebt_f0a20fea-3c9c-4968-a694-67cc4752fbc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromRepaymentsOfShortTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_ProceedsFromRepaymentsOfShortTermDebt_f0a20fea-3c9c-4968-a694-67cc4752fbc3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividends_e460d47f-e42e-46fd-823f-9c6b628025a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_PaymentsOfDividends_e460d47f-e42e-46fd-823f-9c6b628025a9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_577f5dd0-4809-4b32-b08d-3e37b5c0c08c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_577f5dd0-4809-4b32-b08d-3e37b5c0c08c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_dbfccc22-f23c-4300-bbfd-8e4ac4fbcde4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_dbfccc22-f23c-4300-bbfd-8e4ac4fbcde4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_77bdc861-80f1-4e73-b34a-54c43ef6d756" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_b315e5dc-b928-485a-8b5d-d62ac9fa0cc2" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_77bdc861-80f1-4e73-b34a-54c43ef6d756" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_a9028ff3-a5d5-466b-934f-b2a3605fdb1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_90519cdf-60f3-4b1a-a17e-19d71de7e39b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_90519cdf-60f3-4b1a-a17e-19d71de7e39b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_3cc94781-8937-4eba-b9cc-10dd25a60991" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_90519cdf-60f3-4b1a-a17e-19d71de7e39b" xlink:to="loc_us-gaap_InterestPaidNet_3cc94781-8937-4eba-b9cc-10dd25a60991" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNetAbstract_019c6564-325f-7dd1-a425-e87db97dec23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_90519cdf-60f3-4b1a-a17e-19d71de7e39b" xlink:to="loc_us-gaap_IncomeTaxesPaidNetAbstract_019c6564-325f-7dd1-a425-e87db97dec23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019c56c9-3f91-7087-a26d-96791ad62467" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidFederalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxesPaidNetAbstract_019c6564-325f-7dd1-a425-e87db97dec23" xlink:to="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019c56c9-3f91-7087-a26d-96791ad62467" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_IncomeTaxPaidFederalEnergyRelatedTaxCreditsAfterRefundReceived_019c6560-6323-72be-9c1b-17b21c0c3b4a" xlink:href="fsbc-20251231.xsd#fsbc_IncomeTaxPaidFederalEnergyRelatedTaxCreditsAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxesPaidNetAbstract_019c6564-325f-7dd1-a425-e87db97dec23" xlink:to="loc_fsbc_IncomeTaxPaidFederalEnergyRelatedTaxCreditsAfterRefundReceived_019c6560-6323-72be-9c1b-17b21c0c3b4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract_019c6563-1cb3-70a1-a8d1-4561ebcbcb0e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxesPaidNetAbstract_019c6564-325f-7dd1-a425-e87db97dec23" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract_019c6563-1cb3-70a1-a8d1-4561ebcbcb0e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019c56c9-3f91-71ea-a452-b14164522cdd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract_019c6563-1cb3-70a1-a8d1-4561ebcbcb0e" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019c56c9-3f91-71ea-a452-b14164522cdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_459baea8-cd07-4dac-8966-20359d7165df" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxesPaidNetAbstract_019c6564-325f-7dd1-a425-e87db97dec23" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_459baea8-cd07-4dac-8966-20359d7165df" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncashInvestingAndFinancingItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b574db85-7471-4001-9a07-adf5ae17f7f7" xlink:to="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TransferFromLoansHeldForSaleToLoansHeldForInvestment_4e9fdc11-b207-422e-b595-aaeae37d3182" xlink:href="fsbc-20251231.xsd#fsbc_TransferFromLoansHeldForSaleToLoansHeldForInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:to="loc_fsbc_TransferFromLoansHeldForSaleToLoansHeldForInvestment_4e9fdc11-b207-422e-b595-aaeae37d3182" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossOnInvestments_f7702f9f-81fb-4d44-8d62-eea445fdc8e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossOnInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:to="loc_us-gaap_UnrealizedGainLossOnInvestments_f7702f9f-81fb-4d44-8d62-eea445fdc8e9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalLeaseObligationsIncurred_8b72b11d-a1ac-414b-b638-c5802adb5109" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalLeaseObligationsIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:to="loc_us-gaap_CapitalLeaseObligationsIncurred_8b72b11d-a1ac-414b-b638-c5802adb5109" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_NoncashTransactionToAcquireOperatingLeaseRightOfUseAsset_51055a34-598f-4740-abea-a8a515f2f7ff" xlink:href="fsbc-20251231.xsd#fsbc_NoncashTransactionToAcquireOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:to="loc_fsbc_NoncashTransactionToAcquireOperatingLeaseRightOfUseAsset_51055a34-598f-4740-abea-a8a515f2f7ff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_IncreaseDecreaseInCommitmentForLowIncomeHousingTaxCredits_c58f84f0-cbab-48a7-9789-2152ea30ef02" xlink:href="fsbc-20251231.xsd#fsbc_IncreaseDecreaseInCommitmentForLowIncomeHousingTaxCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:to="loc_fsbc_IncreaseDecreaseInCommitmentForLowIncomeHousingTaxCredits_c58f84f0-cbab-48a7-9789-2152ea30ef02" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_4951135f-6373-4855-b62c-adf55eb29f66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_60198d56-720c-4add-a9e8-9ede388575c2" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_4951135f-6373-4855-b62c-adf55eb29f66" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/BasisofPresentationScheduleofUsefulLivesDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#BasisofPresentationScheduleofUsefulLivesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/BasisofPresentationScheduleofUsefulLivesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_05a3bed1-9a51-4cab-849f-020e320147c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3f65c9a9-59e7-418f-ace6-c541d797c3a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_05a3bed1-9a51-4cab-849f-020e320147c6" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3f65c9a9-59e7-418f-ace6-c541d797c3a2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1b332e3e-6cb2-4749-b7fc-f0d35d05e538" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3f65c9a9-59e7-418f-ace6-c541d797c3a2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1b332e3e-6cb2-4749-b7fc-f0d35d05e538" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1b332e3e-6cb2-4749-b7fc-f0d35d05e538_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1b332e3e-6cb2-4749-b7fc-f0d35d05e538" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1b332e3e-6cb2-4749-b7fc-f0d35d05e538_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_88f26c70-14ec-4800-802c-c943fbd1f588" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1b332e3e-6cb2-4749-b7fc-f0d35d05e538" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_88f26c70-14ec-4800-802c-c943fbd1f588" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_f98a0f93-ac8e-4f11-802e-5ef19723b6c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_88f26c70-14ec-4800-802c-c943fbd1f588" xlink:to="loc_us-gaap_EquipmentMember_f98a0f93-ac8e-4f11-802e-5ef19723b6c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_03824cf0-2436-4e15-9e7f-666f7615108d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_88f26c70-14ec-4800-802c-c943fbd1f588" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_03824cf0-2436-4e15-9e7f-666f7615108d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_ed93e7f4-c42a-46bb-870e-67bcd3f9e35d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_88f26c70-14ec-4800-802c-c943fbd1f588" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_ed93e7f4-c42a-46bb-870e-67bcd3f9e35d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AutomobilesMember_26bbc1c5-81c6-451d-a0fd-6f5f83f7a4bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AutomobilesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_88f26c70-14ec-4800-802c-c943fbd1f588" xlink:to="loc_us-gaap_AutomobilesMember_26bbc1c5-81c6-451d-a0fd-6f5f83f7a4bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_8b1b6502-c98e-4236-9457-accd863ce4ee" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3f65c9a9-59e7-418f-ace6-c541d797c3a2" xlink:to="loc_srt_RangeAxis_8b1b6502-c98e-4236-9457-accd863ce4ee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_8b1b6502-c98e-4236-9457-accd863ce4ee_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_8b1b6502-c98e-4236-9457-accd863ce4ee" xlink:to="loc_srt_RangeMember_8b1b6502-c98e-4236-9457-accd863ce4ee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a45057bc-9bf8-4a42-8fe5-17e576c888df" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_8b1b6502-c98e-4236-9457-accd863ce4ee" xlink:to="loc_srt_RangeMember_a45057bc-9bf8-4a42-8fe5-17e576c888df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_81a7a6fd-9177-47d9-8c12-6896b55d05ea" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a45057bc-9bf8-4a42-8fe5-17e576c888df" xlink:to="loc_srt_MinimumMember_81a7a6fd-9177-47d9-8c12-6896b55d05ea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_3e28ec92-fd6c-4c85-a2af-69231c992b7a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a45057bc-9bf8-4a42-8fe5-17e576c888df" xlink:to="loc_srt_MaximumMember_3e28ec92-fd6c-4c85-a2af-69231c992b7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_c8ba0ed6-4af0-42eb-83bd-d9d320196407" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_05a3bed1-9a51-4cab-849f-020e320147c6" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_c8ba0ed6-4af0-42eb-83bd-d9d320196407" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/RecentlyIssuedAccountingStandardsScheduleofCumulativeEffectAdjustmentforAdoptionofASC326Details" xlink:type="simple" xlink:href="fsbc-20251231.xsd#RecentlyIssuedAccountingStandardsScheduleofCumulativeEffectAdjustmentforAdoptionofASC326Details"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/RecentlyIssuedAccountingStandardsScheduleofCumulativeEffectAdjustmentforAdoptionofASC326Details" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ee6a95f3-6056-45f5-96e2-eb7422a241e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_78e9ac03-786a-4eaf-8e3f-50007fe548cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ee6a95f3-6056-45f5-96e2-eb7422a241e9" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_78e9ac03-786a-4eaf-8e3f-50007fe548cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_42710e6b-7011-4aad-aaf6-36c455fab820" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_78e9ac03-786a-4eaf-8e3f-50007fe548cb" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_42710e6b-7011-4aad-aaf6-36c455fab820" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_42710e6b-7011-4aad-aaf6-36c455fab820_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_42710e6b-7011-4aad-aaf6-36c455fab820" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_42710e6b-7011-4aad-aaf6-36c455fab820_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c683692c-c43c-4c21-be7a-546fb34241b1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_42710e6b-7011-4aad-aaf6-36c455fab820" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c683692c-c43c-4c21-be7a-546fb34241b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_238db0bf-89aa-47d6-811c-b705c7e06e45" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c683692c-c43c-4c21-be7a-546fb34241b1" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_238db0bf-89aa-47d6-811c-b705c7e06e45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember_e89c222a-4a72-415d-ac9c-035cc13b78b5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c683692c-c43c-4c21-be7a-546fb34241b1" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember_e89c222a-4a72-415d-ac9c-035cc13b78b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_fb800b09-508a-43e8-a359-fc3d67435979" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_78e9ac03-786a-4eaf-8e3f-50007fe548cb" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_fb800b09-508a-43e8-a359-fc3d67435979" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fb800b09-508a-43e8-a359-fc3d67435979_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_fb800b09-508a-43e8-a359-fc3d67435979" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fb800b09-508a-43e8-a359-fc3d67435979_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_bb381c04-f378-4679-8ce8-a48f641e89fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_fb800b09-508a-43e8-a359-fc3d67435979" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_bb381c04-f378-4679-8ce8-a48f641e89fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedLoanCommitmentMember_2f99a94d-9ee8-40ec-a7ff-de70b77dfa89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_bb381c04-f378-4679-8ce8-a48f641e89fe" xlink:to="loc_us-gaap_UnfundedLoanCommitmentMember_2f99a94d-9ee8-40ec-a7ff-de70b77dfa89" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_674f43a8-18fc-4197-958e-e74beb8eccb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ee6a95f3-6056-45f5-96e2-eb7422a241e9" xlink:to="loc_us-gaap_AssetsAbstract_674f43a8-18fc-4197-958e-e74beb8eccb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_4d83c66a-867c-4d51-9295-73ad8c761d12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_674f43a8-18fc-4197-958e-e74beb8eccb6" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_4d83c66a-867c-4d51-9295-73ad8c761d12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_66d63d1f-24e7-4464-b76a-1cc2c966c1f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_674f43a8-18fc-4197-958e-e74beb8eccb6" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_66d63d1f-24e7-4464-b76a-1cc2c966c1f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOther_c34e732d-f62e-4df6-85a9-b4e67825dd60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_674f43a8-18fc-4197-958e-e74beb8eccb6" xlink:to="loc_us-gaap_DeferredTaxAssetsOther_c34e732d-f62e-4df6-85a9-b4e67825dd60" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_3da007e9-f461-495b-962f-4fb85e9165d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ee6a95f3-6056-45f5-96e2-eb7422a241e9" xlink:to="loc_us-gaap_LiabilitiesAbstract_3da007e9-f461-495b-962f-4fb85e9165d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_402dc0d4-66b1-44da-8401-9fe5ca22983d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityAbstract_1c8b304c-825b-4d4f-a9ca-b107b0c1ccf5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_ee6a95f3-6056-45f5-96e2-eb7422a241e9" xlink:to="loc_us-gaap_EquityAbstract_1c8b304c-825b-4d4f-a9ca-b107b0c1ccf5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_27a0f596-c4b8-4bb5-97b2-ca4ae62f0f8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityAbstract_1c8b304c-825b-4d4f-a9ca-b107b0c1ccf5" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_27a0f596-c4b8-4bb5-97b2-ca4ae62f0f8d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAmortizedCostandFairValueofSecuritiesHeldtomaturityDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InvestmentSecuritiesScheduleofAmortizedCostandFairValueofSecuritiesHeldtomaturityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAmortizedCostandFairValueofSecuritiesHeldtomaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_5ceccf60-2da6-4c54-a1c1-683469f648b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:to="loc_us-gaap_MarketableSecuritiesTable_5ceccf60-2da6-4c54-a1c1-683469f648b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_a6ae98a8-132b-4ece-aaad-614f147075b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_5ceccf60-2da6-4c54-a1c1-683469f648b5" xlink:to="loc_us-gaap_FinancialInstrumentAxis_a6ae98a8-132b-4ece-aaad-614f147075b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a6ae98a8-132b-4ece-aaad-614f147075b0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_a6ae98a8-132b-4ece-aaad-614f147075b0" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a6ae98a8-132b-4ece-aaad-614f147075b0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_168107fc-7788-4e6c-b514-23ba5d3d5a6d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_a6ae98a8-132b-4ece-aaad-614f147075b0" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_168107fc-7788-4e6c-b514-23ba5d3d5a6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_4645ba4a-c44f-4b78-b326-71b1eb77b526" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_168107fc-7788-4e6c-b514-23ba5d3d5a6d" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_4645ba4a-c44f-4b78-b326-71b1eb77b526" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_6366f102-be90-4592-840c-a8a0be6d8530" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_6366f102-be90-4592-840c-a8a0be6d8530" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecurities_026a2d29-6097-4fe2-90b9-e9f8ae8555f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:to="loc_us-gaap_HeldToMaturitySecurities_026a2d29-6097-4fe2-90b9-e9f8ae8555f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain_56bf133c-0902-43e7-b6c1-4487e6f12e62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain_56bf133c-0902-43e7-b6c1-4487e6f12e62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss_b8b8422e-8135-44bb-9f91-652b921e94e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss_b8b8422e-8135-44bb-9f91-652b921e94e3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_6da517b6-47d2-46c1-8ba5-d842d9e83fb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_1691e555-d523-43ea-b4e1-3c6a1a0e2782" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_6da517b6-47d2-46c1-8ba5-d842d9e83fb0" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InvestmentSecuritiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_2e6ad64a-15b3-4c55-b045-4bf0df654eaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_MarketableSecuritiesTable_2e6ad64a-15b3-4c55-b045-4bf0df654eaf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_9cb4dda9-ef68-4b32-a359-55e796c41487" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_2e6ad64a-15b3-4c55-b045-4bf0df654eaf" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_9cb4dda9-ef68-4b32-a359-55e796c41487" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9cb4dda9-ef68-4b32-a359-55e796c41487_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_9cb4dda9-ef68-4b32-a359-55e796c41487" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9cb4dda9-ef68-4b32-a359-55e796c41487_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_808bb5cd-6964-452f-9207-2e5f4c5759fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_9cb4dda9-ef68-4b32-a359-55e796c41487" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_808bb5cd-6964-452f-9207-2e5f4c5759fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentInFederalHomeLoanBankStockMember_16ca3acb-a582-4849-9047-19e2e43b70b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentInFederalHomeLoanBankStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_808bb5cd-6964-452f-9207-2e5f4c5759fb" xlink:to="loc_us-gaap_InvestmentInFederalHomeLoanBankStockMember_16ca3acb-a582-4849-9047-19e2e43b70b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_dfbcc44c-52fd-4056-bce8-df0a3f13d10e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_dfbcc44c-52fd-4056-bce8-df0a3f13d10e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_278bcba2-4e30-4c22-a885-c624bdcc0491" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_278bcba2-4e30-4c22-a885-c624bdcc0491" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions_f104132f-afa1-4e80-b46b-bf14a8412c31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions_f104132f-afa1-4e80-b46b-bf14a8412c31" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions_0d87e1c5-38c6-4c9b-b5e5-548ac2fcdc70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions_0d87e1c5-38c6-4c9b-b5e5-548ac2fcdc70" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions_388ab381-be8e-45b0-a480-7677dbf8ed5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions_388ab381-be8e-45b0-a480-7677dbf8ed5b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionMoreThan12MonthsNumberOfPositions_bf77a0dc-64cb-4ff2-985b-256d52a8cdbb" xlink:href="fsbc-20251231.xsd#fsbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionMoreThan12MonthsNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_fsbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionMoreThan12MonthsNumberOfPositions_bf77a0dc-64cb-4ff2-985b-256d52a8cdbb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalHomeLoanBankAdvancesStockPriceForSalesAndPurchases_96580472-e0b8-4dcf-a1ea-55c73d5b10b5" xlink:href="fsbc-20251231.xsd#fsbc_FederalHomeLoanBankAdvancesStockPriceForSalesAndPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_fsbc_FederalHomeLoanBankAdvancesStockPriceForSalesAndPurchases_96580472-e0b8-4dcf-a1ea-55c73d5b10b5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalHomeLoanBankStock_83eb9dfa-d750-464c-b602-9985ab194342" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_FederalHomeLoanBankStock_83eb9dfa-d750-464c-b602-9985ab194342" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalHomeLoanBankStockDividends_a4442115-470f-4bda-b3f2-54d34cd5f5c4" xlink:href="fsbc-20251231.xsd#fsbc_FederalHomeLoanBankStockDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_fsbc_FederalHomeLoanBankStockDividends_a4442115-470f-4bda-b3f2-54d34cd5f5c4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019b2682-08f7-7e7c-8f5a-9a733314bdb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_EquityMethodInvestments_019b2682-08f7-7e7c-8f5a-9a733314bdb2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal_019c6ff0-6db0-7143-88aa-9f9a75f1be4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_f61bee9a-2e45-49a5-837d-9b71e4f98883" xlink:to="loc_us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal_019c6ff0-6db0-7143-88aa-9f9a75f1be4a" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAmortizedCostandFairValueofSecuritiesAvailableforsaleDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InvestmentSecuritiesScheduleofAmortizedCostandFairValueofSecuritiesAvailableforsaleDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAmortizedCostandFairValueofSecuritiesAvailableforsaleDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_5f785dba-42fc-4d0c-82c9-a82f2ee8d86b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_25fa4c00-c0c5-427a-a5ba-077a3e061213" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_5f785dba-42fc-4d0c-82c9-a82f2ee8d86b" xlink:to="loc_us-gaap_MarketableSecuritiesTable_25fa4c00-c0c5-427a-a5ba-077a3e061213" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_156aeaf9-9e07-4d42-833a-1828a7a35ad0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_25fa4c00-c0c5-427a-a5ba-077a3e061213" xlink:to="loc_us-gaap_FinancialInstrumentAxis_156aeaf9-9e07-4d42-833a-1828a7a35ad0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_156aeaf9-9e07-4d42-833a-1828a7a35ad0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_156aeaf9-9e07-4d42-833a-1828a7a35ad0" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_156aeaf9-9e07-4d42-833a-1828a7a35ad0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_156aeaf9-9e07-4d42-833a-1828a7a35ad0" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_ef85b096-3dc4-44cd-9b52-b21d8f952bb4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_ef85b096-3dc4-44cd-9b52-b21d8f952bb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_85e23ed5-9633-4abd-9838-c51820bbe0af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_85e23ed5-9633-4abd-9838-c51820bbe0af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_3b1b5fcf-decb-41da-894a-157d4116081a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_3b1b5fcf-decb-41da-894a-157d4116081a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedDebtObligationsMember_86fbbd53-1f6d-4790-9bfe-030745937b33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollateralizedDebtObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:to="loc_us-gaap_CollateralizedDebtObligationsMember_86fbbd53-1f6d-4790-9bfe-030745937b33" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_28a842ff-5963-487b-bef3-1b486a0712fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7ecf52cc-d480-4ba3-9929-629f507f0cf3" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_28a842ff-5963-487b-bef3-1b486a0712fb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_f32fa77a-f565-48ef-a2ad-e05443f53ccf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_5f785dba-42fc-4d0c-82c9-a82f2ee8d86b" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_f32fa77a-f565-48ef-a2ad-e05443f53ccf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_fb492ee3-8bd1-4d44-b3e8-81f2fe519446" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_5f785dba-42fc-4d0c-82c9-a82f2ee8d86b" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_fb492ee3-8bd1-4d44-b3e8-81f2fe519446" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_d59dc33a-dc02-431e-859d-d35e6508c40c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_5f785dba-42fc-4d0c-82c9-a82f2ee8d86b" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_d59dc33a-dc02-431e-859d-d35e6508c40c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_673cfc54-9b43-412e-a19b-3341628040c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_5f785dba-42fc-4d0c-82c9-a82f2ee8d86b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_673cfc54-9b43-412e-a19b-3341628040c1" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAmortizedCostandFairValueofInvestmentDebtSecuritiesbyContractualMaturityDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InvestmentSecuritiesScheduleofAmortizedCostandFairValueofInvestmentDebtSecuritiesbyContractualMaturityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAmortizedCostandFairValueofInvestmentDebtSecuritiesbyContractualMaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_ffe93a7a-b7e0-49c4-9739-bff29588efab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_MarketableSecuritiesTable_ffe93a7a-b7e0-49c4-9739-bff29588efab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_b5d28b7e-dc85-4432-a6e3-9c02d218569d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_ffe93a7a-b7e0-49c4-9739-bff29588efab" xlink:to="loc_us-gaap_FinancialInstrumentAxis_b5d28b7e-dc85-4432-a6e3-9c02d218569d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_b5d28b7e-dc85-4432-a6e3-9c02d218569d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_b5d28b7e-dc85-4432-a6e3-9c02d218569d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_b5d28b7e-dc85-4432-a6e3-9c02d218569d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3b8b528e-a2dd-4ed4-8468-cddc2fed5f70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_b5d28b7e-dc85-4432-a6e3-9c02d218569d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3b8b528e-a2dd-4ed4-8468-cddc2fed5f70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_5bccaecc-63f5-4a05-ae2d-c1e36fea6050" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3b8b528e-a2dd-4ed4-8468-cddc2fed5f70" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_5bccaecc-63f5-4a05-ae2d-c1e36fea6050" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_5760b43f-8ea6-4ee3-bb63-09bf91b7c639" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3b8b528e-a2dd-4ed4-8468-cddc2fed5f70" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_5760b43f-8ea6-4ee3-bb63-09bf91b7c639" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedDebtObligationsMember_e555b3a7-d5b8-4d85-a368-47abe448a8f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollateralizedDebtObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3b8b528e-a2dd-4ed4-8468-cddc2fed5f70" xlink:to="loc_us-gaap_CollateralizedDebtObligationsMember_e555b3a7-d5b8-4d85-a368-47abe448a8f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_85d38e64-6fa5-4b1a-a4f4-f266731dd192" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3b8b528e-a2dd-4ed4-8468-cddc2fed5f70" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_85d38e64-6fa5-4b1a-a4f4-f266731dd192" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_1372faca-084a-4da2-bfcf-3213409fc817" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_1372faca-084a-4da2-bfcf-3213409fc817" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_889fb97a-1425-49ca-a139-d836c40b753b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_1372faca-084a-4da2-bfcf-3213409fc817" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_889fb97a-1425-49ca-a139-d836c40b753b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount_0ac9ca3f-cdcb-48fd-9283-c18cd9f64257" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_889fb97a-1425-49ca-a139-d836c40b753b" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount_0ac9ca3f-cdcb-48fd-9283-c18cd9f64257" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount_8455a1a7-6f3e-4b5d-b186-4f288b348d28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_889fb97a-1425-49ca-a139-d836c40b753b" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount_8455a1a7-6f3e-4b5d-b186-4f288b348d28" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount_6cafa884-51fd-4b1b-9b86-6dc5bc153d0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_889fb97a-1425-49ca-a139-d836c40b753b" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount_6cafa884-51fd-4b1b-9b86-6dc5bc153d0d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount_233a9fbe-bc5b-462a-863f-00c7a7df7d6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_889fb97a-1425-49ca-a139-d836c40b753b" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount_233a9fbe-bc5b-462a-863f-00c7a7df7d6a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_48937916-69fa-495e-b9eb-fcae465d837f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_1372faca-084a-4da2-bfcf-3213409fc817" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_48937916-69fa-495e-b9eb-fcae465d837f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue_3cd23706-4046-4062-bd5f-17a775ecc46f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_48937916-69fa-495e-b9eb-fcae465d837f" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue_3cd23706-4046-4062-bd5f-17a775ecc46f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_5354bf60-32a9-444c-a367-b5800ba9625c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_48937916-69fa-495e-b9eb-fcae465d837f" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_5354bf60-32a9-444c-a367-b5800ba9625c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_dbda555d-1f33-4e45-b3e8-872551a768a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_48937916-69fa-495e-b9eb-fcae465d837f" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_dbda555d-1f33-4e45-b3e8-872551a768a7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue_ba0f72bc-a28b-42ee-a68d-a8cfbee528aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_48937916-69fa-495e-b9eb-fcae465d837f" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue_ba0f72bc-a28b-42ee-a68d-a8cfbee528aa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_756c97bf-4875-4075-bb51-402030d336b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_756c97bf-4875-4075-bb51-402030d336b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_bbaa0a69-efbb-4fab-8ca0-7554c34a2996" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_756c97bf-4875-4075-bb51-402030d336b5" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_bbaa0a69-efbb-4fab-8ca0-7554c34a2996" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_00d550d6-2fd2-44a0-a6b2-cfa2f1701181" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_bbaa0a69-efbb-4fab-8ca0-7554c34a2996" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_00d550d6-2fd2-44a0-a6b2-cfa2f1701181" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_c82992e0-edca-4834-8d5c-fb3b8cf2c35c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_bbaa0a69-efbb-4fab-8ca0-7554c34a2996" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_c82992e0-edca-4834-8d5c-fb3b8cf2c35c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_5fecff0d-048c-44b5-b0fb-15680985a072" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_bbaa0a69-efbb-4fab-8ca0-7554c34a2996" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_5fecff0d-048c-44b5-b0fb-15680985a072" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost_793a6373-9e95-47ea-b3bd-dba4488b0bbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_bbaa0a69-efbb-4fab-8ca0-7554c34a2996" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost_793a6373-9e95-47ea-b3bd-dba4488b0bbd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_d3052e1f-03f9-416f-ba2b-aa41299adc23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_756c97bf-4875-4075-bb51-402030d336b5" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_d3052e1f-03f9-416f-ba2b-aa41299adc23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_7cedcc61-af10-429f-8dd7-1ef02edf44ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_d3052e1f-03f9-416f-ba2b-aa41299adc23" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_7cedcc61-af10-429f-8dd7-1ef02edf44ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_74979124-eb68-4c17-b329-e211c0bcc382" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_d3052e1f-03f9-416f-ba2b-aa41299adc23" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_74979124-eb68-4c17-b329-e211c0bcc382" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_a1ce8f5d-c3b9-417f-993a-d52f9b6dc708" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_d3052e1f-03f9-416f-ba2b-aa41299adc23" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_a1ce8f5d-c3b9-417f-993a-d52f9b6dc708" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue_01efe6f8-3705-4719-863a-b4106d82cf34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_d3052e1f-03f9-416f-ba2b-aa41299adc23" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue_01efe6f8-3705-4719-863a-b4106d82cf34" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecurities_aafea4fd-b660-4b7c-a3a5-c1bf716e7bf7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_HeldToMaturitySecurities_aafea4fd-b660-4b7c-a3a5-c1bf716e7bf7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_8bd91592-d245-4d32-aac8-c486c6bb142a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_8bd91592-d245-4d32-aac8-c486c6bb142a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_e3ac83ec-069b-4676-bed2-5795ff6bb5f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_e3ac83ec-069b-4676-bed2-5795ff6bb5f8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_41bb5b02-dac2-4413-b134-bc9e87df6dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_8ac95505-dbb9-4a4a-886c-9aae261270a4" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_41bb5b02-dac2-4413-b134-bc9e87df6dbf" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofPledgedInvestmentSecuritiesDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InvestmentSecuritiesScheduleofPledgedInvestmentSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofPledgedInvestmentSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_862b6456-ea7e-43a6-8a74-67a662657ab9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_9cbebdaf-0786-44dc-b75d-5ff72adbfc28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_862b6456-ea7e-43a6-8a74-67a662657ab9" xlink:to="loc_us-gaap_MarketableSecuritiesTable_9cbebdaf-0786-44dc-b75d-5ff72adbfc28" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_f3ec7fcf-52cc-4dac-be37-e7c6b83882f0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_9cbebdaf-0786-44dc-b75d-5ff72adbfc28" xlink:to="loc_srt_StatementGeographicalAxis_f3ec7fcf-52cc-4dac-be37-e7c6b83882f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_f3ec7fcf-52cc-4dac-be37-e7c6b83882f0_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_f3ec7fcf-52cc-4dac-be37-e7c6b83882f0" xlink:to="loc_srt_SegmentGeographicalDomain_f3ec7fcf-52cc-4dac-be37-e7c6b83882f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_53c7b9f6-1348-4fe5-bef9-3359b8c03fc5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_f3ec7fcf-52cc-4dac-be37-e7c6b83882f0" xlink:to="loc_srt_SegmentGeographicalDomain_53c7b9f6-1348-4fe5-bef9-3359b8c03fc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_273042ed-a8fd-479e-8472-fc25b8fa753e" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_53c7b9f6-1348-4fe5-bef9-3359b8c03fc5" xlink:to="loc_stpr_CA_273042ed-a8fd-479e-8472-fc25b8fa753e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgingPurposeAxis_5a327dc2-bd19-40af-b2fe-fe13642608d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_9cbebdaf-0786-44dc-b75d-5ff72adbfc28" xlink:to="loc_us-gaap_PledgingPurposeAxis_5a327dc2-bd19-40af-b2fe-fe13642608d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgingPurposeDomain_5a327dc2-bd19-40af-b2fe-fe13642608d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PledgingPurposeAxis_5a327dc2-bd19-40af-b2fe-fe13642608d1" xlink:to="loc_us-gaap_PledgingPurposeDomain_5a327dc2-bd19-40af-b2fe-fe13642608d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgingPurposeDomain_44eff965-6961-461e-989e-c66c5e7a5c32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PledgingPurposeAxis_5a327dc2-bd19-40af-b2fe-fe13642608d1" xlink:to="loc_us-gaap_PledgingPurposeDomain_44eff965-6961-461e-989e-c66c5e7a5c32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepositsMember_20f415fb-96ae-42cb-9976-ad0916376889" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgingPurposeDomain_44eff965-6961-461e-989e-c66c5e7a5c32" xlink:to="loc_us-gaap_DepositsMember_20f415fb-96ae-42cb-9976-ad0916376889" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalFundsPurchasedMember_80534e5e-a373-4930-bd38-c00378595c96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalFundsPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgingPurposeDomain_44eff965-6961-461e-989e-c66c5e7a5c32" xlink:to="loc_us-gaap_FederalFundsPurchasedMember_80534e5e-a373-4930-bd38-c00378595c96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DepositsAndFederalFundsPurchasedMember_8c268bc4-d0b8-493a-81f8-63df1ab112bd" xlink:href="fsbc-20251231.xsd#fsbc_DepositsAndFederalFundsPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgingPurposeDomain_44eff965-6961-461e-989e-c66c5e7a5c32" xlink:to="loc_fsbc_DepositsAndFederalFundsPurchasedMember_8c268bc4-d0b8-493a-81f8-63df1ab112bd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_ae3ce23e-15d6-479d-b515-6a3a3c86ec2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_862b6456-ea7e-43a6-8a74-67a662657ab9" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_ae3ce23e-15d6-479d-b515-6a3a3c86ec2a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAvailableforsaleSecuritiesinaContinuousUnrealizedLossPositionDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InvestmentSecuritiesScheduleofAvailableforsaleSecuritiesinaContinuousUnrealizedLossPositionDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesScheduleofAvailableforsaleSecuritiesinaContinuousUnrealizedLossPositionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_6700d3c8-8c10-4615-92ba-606dce509fc2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_us-gaap_MarketableSecuritiesTable_6700d3c8-8c10-4615-92ba-606dce509fc2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_3d0668d3-062b-4b3d-bf0a-5141b38234dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_6700d3c8-8c10-4615-92ba-606dce509fc2" xlink:to="loc_us-gaap_FinancialInstrumentAxis_3d0668d3-062b-4b3d-bf0a-5141b38234dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3d0668d3-062b-4b3d-bf0a-5141b38234dd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_3d0668d3-062b-4b3d-bf0a-5141b38234dd" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3d0668d3-062b-4b3d-bf0a-5141b38234dd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_3d0668d3-062b-4b3d-bf0a-5141b38234dd" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_73af3456-a3a0-44cf-8894-a2ba7a5db4bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_73af3456-a3a0-44cf-8894-a2ba7a5db4bb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_72e7cc21-e807-46c6-aa9d-364c5f9db60a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_72e7cc21-e807-46c6-aa9d-364c5f9db60a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_dcb9cfbe-ef8c-48e8-9ba1-a7c07fe663c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_dcb9cfbe-ef8c-48e8-9ba1-a7c07fe663c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedDebtObligationsMember_fba84b6b-6971-4bc3-8bcc-950baa92cf26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollateralizedDebtObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:to="loc_us-gaap_CollateralizedDebtObligationsMember_fba84b6b-6971-4bc3-8bcc-950baa92cf26" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_23fe652a-5291-423e-b4a5-bebf516f27c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_00aa1fd9-59ab-45a5-87b8-36cceab7a57a" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_23fe652a-5291-423e-b4a5-bebf516f27c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_5d88d30f-f708-44f2-8e1f-4f28d471f60e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_5d88d30f-f708-44f2-8e1f-4f28d471f60e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_7fd1121c-48dd-4f6b-90dd-7c5ddee6938e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_7fd1121c-48dd-4f6b-90dd-7c5ddee6938e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerFairValue_76536142-10cf-4a37-acf3-7e63fe74dbbf" xlink:href="fsbc-20251231.xsd#fsbc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_fsbc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerFairValue_76536142-10cf-4a37-acf3-7e63fe74dbbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_0ca867a6-c4e5-44e6-bffd-870cabfa0d05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_0ca867a6-c4e5-44e6-bffd-870cabfa0d05" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValue_79df9e61-c81f-489d-adaa-ccf8a8b83944" xlink:href="fsbc-20251231.xsd#fsbc_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_fsbc_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValue_79df9e61-c81f-489d-adaa-ccf8a8b83944" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_c01353b8-336b-412f-b6cd-f4955114db3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_81c299ca-2a9a-4246-8bd0-f7d52465b97f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_c01353b8-336b-412f-b6cd-f4955114db3d" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofBalanceofEachMajorProductTypeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofBalanceofEachMajorProductTypeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofBalanceofEachMajorProductTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_d94d454c-474e-481d-8ec1-10b9d584d632" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_d94d454c-474e-481d-8ec1-10b9d584d632" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_9826c716-350e-4b94-a4c6-402b89aff7de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_d94d454c-474e-481d-8ec1-10b9d584d632" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_9826c716-350e-4b94-a4c6-402b89aff7de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_9826c716-350e-4b94-a4c6-402b89aff7de_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_9826c716-350e-4b94-a4c6-402b89aff7de" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_9826c716-350e-4b94-a4c6-402b89aff7de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_9826c716-350e-4b94-a4c6-402b89aff7de" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_4944b643-7343-4c17-a16c-99e4dc02eb40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_us-gaap_CommercialRealEstateMember_4944b643-7343-4c17-a16c-99e4dc02eb40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_ac705d5d-db08-4288-96a1-b9c193382880" xlink:href="fsbc-20251231.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_ac705d5d-db08-4288-96a1-b9c193382880" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_8a657ab2-b306-4214-9953-bd04acd3e8f6" xlink:href="fsbc-20251231.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_fsbc_CommercialConstructionLoansMember_8a657ab2-b306-4214-9953-bd04acd3e8f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_1fc2fa6b-32ad-479b-ae7b-d3fc177cbb2a" xlink:href="fsbc-20251231.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_1fc2fa6b-32ad-479b-ae7b-d3fc177cbb2a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_632bbbdb-b6b6-41c5-9162-596b5b4bd57d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_us-gaap_ResidentialRealEstateMember_632bbbdb-b6b6-41c5-9162-596b5b4bd57d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_b1eea6f7-f0bf-4c96-b59d-148e380e163f" xlink:href="fsbc-20251231.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_fsbc_FarmlandMember_b1eea6f7-f0bf-4c96-b59d-148e380e163f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_8f8fb7b1-2adb-4919-8211-3abee7095722" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_fsbc_SecuredCommercialLoansMember_8f8fb7b1-2adb-4919-8211-3abee7095722" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_820cb836-0526-41ca-b9df-e87fce8e9569" xlink:href="fsbc-20251231.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_820cb836-0526-41ca-b9df-e87fce8e9569" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_0f3c9565-796a-4c86-9cb9-8b20687c1217" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_fa820c1f-c1a0-470e-af16-399b9e1ef593" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_0f3c9565-796a-4c86-9cb9-8b20687c1217" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableBeforeAllowanceForCreditLossAndFee_411c8b13-21fc-4c96-897a-e2c5c6237005" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableBeforeAllowanceForCreditLossAndFee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:to="loc_us-gaap_FinancingReceivableBeforeAllowanceForCreditLossAndFee_411c8b13-21fc-4c96-897a-e2c5c6237005" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_f4c4b1cc-88b6-4927-badc-fec402b0fd15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:to="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_f4c4b1cc-88b6-4927-badc-fec402b0fd15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_1ac6c88c-95cc-45ea-8917-68115a9864a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:to="loc_us-gaap_NotesReceivableGross_1ac6c88c-95cc-45ea-8917-68115a9864a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_f22f7bfe-019d-4ce8-a0fc-dcb7c48e794f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_f22f7bfe-019d-4ce8-a0fc-dcb7c48e794f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableNet_96672737-3206-46fb-863d-cc78ed05fa19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_cb407225-647f-4f4f-be0f-c8c696bc2e7e" xlink:to="loc_us-gaap_NotesReceivableNet_96672737-3206-46fb-863d-cc78ed05fa19" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1caf66b0-ea2b-48a0-9060-85ffd951180b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1caf66b0-ea2b-48a0-9060-85ffd951180b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_5f672b46-e635-400b-9bf1-982a54be1682" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1caf66b0-ea2b-48a0-9060-85ffd951180b" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_5f672b46-e635-400b-9bf1-982a54be1682" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_5f672b46-e635-400b-9bf1-982a54be1682_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_5f672b46-e635-400b-9bf1-982a54be1682" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_5f672b46-e635-400b-9bf1-982a54be1682_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_073e3c71-6743-47ef-8e21-03cab1703bd6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_5f672b46-e635-400b-9bf1-982a54be1682" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_073e3c71-6743-47ef-8e21-03cab1703bd6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansMember_52e879a5-2abd-4ebb-8ca5-87ef5afbf056" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_073e3c71-6743-47ef-8e21-03cab1703bd6" xlink:to="loc_us-gaap_LoansMember_52e879a5-2abd-4ebb-8ca5-87ef5afbf056" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_ea99719f-496f-42cb-a4b1-f13266037af9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1caf66b0-ea2b-48a0-9060-85ffd951180b" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_ea99719f-496f-42cb-a4b1-f13266037af9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_ea99719f-496f-42cb-a4b1-f13266037af9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_ea99719f-496f-42cb-a4b1-f13266037af9" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_ea99719f-496f-42cb-a4b1-f13266037af9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_9566f641-964b-4758-b27d-aa4a4df01b81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_ea99719f-496f-42cb-a4b1-f13266037af9" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_9566f641-964b-4758-b27d-aa4a4df01b81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_LoanConcentrationRiskMember_369d9127-97ea-49a1-8d16-066af1fe7b58" xlink:href="fsbc-20251231.xsd#fsbc_LoanConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_9566f641-964b-4758-b27d-aa4a4df01b81" xlink:to="loc_fsbc_LoanConcentrationRiskMember_369d9127-97ea-49a1-8d16-066af1fe7b58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_97fc32c0-e993-4fad-a550-636319b1256e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1caf66b0-ea2b-48a0-9060-85ffd951180b" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_97fc32c0-e993-4fad-a550-636319b1256e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_97fc32c0-e993-4fad-a550-636319b1256e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_97fc32c0-e993-4fad-a550-636319b1256e" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_97fc32c0-e993-4fad-a550-636319b1256e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_e916951f-1a98-477e-b8cf-7181e59a192b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_97fc32c0-e993-4fad-a550-636319b1256e" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_e916951f-1a98-477e-b8cf-7181e59a192b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstatePortfolioSegmentMember_4b55c612-ac64-4109-abee-8fd7d5316037" xlink:href="fsbc-20251231.xsd#fsbc_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_e916951f-1a98-477e-b8cf-7181e59a192b" xlink:to="loc_fsbc_RealEstatePortfolioSegmentMember_4b55c612-ac64-4109-abee-8fd7d5316037" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPortfolioSegmentMember_9bd79cca-7f51-467d-85f8-f3deacaa6737" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_e916951f-1a98-477e-b8cf-7181e59a192b" xlink:to="loc_us-gaap_CommercialPortfolioSegmentMember_9bd79cca-7f51-467d-85f8-f3deacaa6737" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ConsumerAndOtherPortfolioSegmentMember_9008048a-0dfe-47c0-9ca5-6fc27fab6926" xlink:href="fsbc-20251231.xsd#fsbc_ConsumerAndOtherPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_e916951f-1a98-477e-b8cf-7181e59a192b" xlink:to="loc_fsbc_ConsumerAndOtherPortfolioSegmentMember_9008048a-0dfe-47c0-9ca5-6fc27fab6926" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankBranchAxis_5d0be500-6e27-46e3-8c70-e9e8425a18ae" xlink:href="fsbc-20251231.xsd#fsbc_FederalReserveBankBranchAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1caf66b0-ea2b-48a0-9060-85ffd951180b" xlink:to="loc_fsbc_FederalReserveBankBranchAxis_5d0be500-6e27-46e3-8c70-e9e8425a18ae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankBranchDomain_5d0be500-6e27-46e3-8c70-e9e8425a18ae_default" xlink:href="fsbc-20251231.xsd#fsbc_FederalReserveBankBranchDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_fsbc_FederalReserveBankBranchAxis_5d0be500-6e27-46e3-8c70-e9e8425a18ae" xlink:to="loc_fsbc_FederalReserveBankBranchDomain_5d0be500-6e27-46e3-8c70-e9e8425a18ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankBranchDomain_22bb5f05-6c33-4fea-be62-ef8140723c1e" xlink:href="fsbc-20251231.xsd#fsbc_FederalReserveBankBranchDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_fsbc_FederalReserveBankBranchAxis_5d0be500-6e27-46e3-8c70-e9e8425a18ae" xlink:to="loc_fsbc_FederalReserveBankBranchDomain_22bb5f05-6c33-4fea-be62-ef8140723c1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankOfSanFranciscoMember_2a6baa7d-dee6-46c9-894e-eea111345efa" xlink:href="fsbc-20251231.xsd#fsbc_FederalReserveBankOfSanFranciscoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FederalReserveBankBranchDomain_22bb5f05-6c33-4fea-be62-ef8140723c1e" xlink:to="loc_fsbc_FederalReserveBankOfSanFranciscoMember_2a6baa7d-dee6-46c9-894e-eea111345efa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_a3c9f7bd-4e85-40f2-8772-e8f6b5e84c19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_a3c9f7bd-4e85-40f2-8772-e8f6b5e84c19" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing_14c20fa1-b432-4c81-bddd-dad653d6d9b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing_14c20fa1-b432-4c81-bddd-dad653d6d9b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageLoansInProcessOfForeclosureAmount_019c9a8f-22c3-70f1-bfab-f824139956c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageLoansInProcessOfForeclosureAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_us-gaap_MortgageLoansInProcessOfForeclosureAmount_019c9a8f-22c3-70f1-bfab-f824139956c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableNonaccrualInterestIncome_cd1d223f-214c-4abc-bc3a-f81786834fcc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableNonaccrualInterestIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_us-gaap_FinancingReceivableNonaccrualInterestIncome_cd1d223f-214c-4abc-bc3a-f81786834fcc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancingReceivableNonaccrualInterestIncomeForegone_38a47ada-fc9e-4694-9d7f-93a01d3cd06c" xlink:href="fsbc-20251231.xsd#fsbc_FinancingReceivableNonaccrualInterestIncomeForegone"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_fsbc_FinancingReceivableNonaccrualInterestIncomeForegone_38a47ada-fc9e-4694-9d7f-93a01d3cd06c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1_10f22980-ec17-4eab-9629-f58defec16b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1_10f22980-ec17-4eab-9629-f58defec16b5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged_6c3d6620-bb57-4bd8-a6eb-f7e819df0058" xlink:href="fsbc-20251231.xsd#fsbc_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_3dd2505e-e8d8-4f43-acc2-d58d67287b43" xlink:to="loc_fsbc_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged_6c3d6620-bb57-4bd8-a6eb-f7e819df0058" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofCreditQualityIndicatorsRelatedtotheCompanysLoansbyClassDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofCreditQualityIndicatorsRelatedtotheCompanysLoansbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofCreditQualityIndicatorsRelatedtotheCompanysLoansbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1a017328-c9cc-4f73-8c4d-3765d2c17c82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1a017328-c9cc-4f73-8c4d-3765d2c17c82" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_a51ae58b-45a1-43c1-ac48-2140a206f124" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1a017328-c9cc-4f73-8c4d-3765d2c17c82" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_a51ae58b-45a1-43c1-ac48-2140a206f124" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_a51ae58b-45a1-43c1-ac48-2140a206f124_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_a51ae58b-45a1-43c1-ac48-2140a206f124" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_a51ae58b-45a1-43c1-ac48-2140a206f124_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_a51ae58b-45a1-43c1-ac48-2140a206f124" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_4979f2ab-574c-42ef-b763-8f9f090fa9ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_us-gaap_CommercialRealEstateMember_4979f2ab-574c-42ef-b763-8f9f090fa9ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_b9d174f0-2247-46e9-9d73-7c64d6df0394" xlink:href="fsbc-20251231.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_b9d174f0-2247-46e9-9d73-7c64d6df0394" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_0d015cb9-9ebf-4449-89c6-759b6ae77012" xlink:href="fsbc-20251231.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_fsbc_CommercialConstructionLoansMember_0d015cb9-9ebf-4449-89c6-759b6ae77012" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_df2b4506-84ba-40c5-a4d4-c2df21b7c0fe" xlink:href="fsbc-20251231.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_df2b4506-84ba-40c5-a4d4-c2df21b7c0fe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_d82ad3bc-6dd9-4ec9-a4f4-5b55a5877967" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_us-gaap_ResidentialRealEstateMember_d82ad3bc-6dd9-4ec9-a4f4-5b55a5877967" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_30a48384-33e9-4136-9e26-2f0e08a0567b" xlink:href="fsbc-20251231.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_fsbc_FarmlandMember_30a48384-33e9-4136-9e26-2f0e08a0567b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_f4ddf045-05c6-47ee-8573-bac4d039592c" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_fsbc_SecuredCommercialLoansMember_f4ddf045-05c6-47ee-8573-bac4d039592c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_6bf89dbe-2ae5-42f4-afca-c9a9109965d7" xlink:href="fsbc-20251231.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_6bf89dbe-2ae5-42f4-afca-c9a9109965d7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_14853345-c642-40ff-b2f6-66ff6a35561c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_167f254d-dec2-464d-bb3b-c51e04d066fa" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_14853345-c642-40ff-b2f6-66ff6a35561c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentAxis_7932b71f-5929-459f-a566-72c62fc9bc8f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_1a017328-c9cc-4f73-8c4d-3765d2c17c82" xlink:to="loc_us-gaap_InternalCreditAssessmentAxis_7932b71f-5929-459f-a566-72c62fc9bc8f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_7932b71f-5929-459f-a566-72c62fc9bc8f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_7932b71f-5929-459f-a566-72c62fc9bc8f" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_7932b71f-5929-459f-a566-72c62fc9bc8f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_3ef69331-670d-4a61-a450-ab3c1b79f043" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_7932b71f-5929-459f-a566-72c62fc9bc8f" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_3ef69331-670d-4a61-a450-ab3c1b79f043" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PassMember_617f9d97-7c98-4a55-8270-5436d302920b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PassMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_3ef69331-670d-4a61-a450-ab3c1b79f043" xlink:to="loc_us-gaap_PassMember_617f9d97-7c98-4a55-8270-5436d302920b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_WatchMember_359bd30d-7104-4159-a92b-411ca133626f" xlink:href="fsbc-20251231.xsd#fsbc_WatchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_3ef69331-670d-4a61-a450-ab3c1b79f043" xlink:to="loc_fsbc_WatchMember_359bd30d-7104-4159-a92b-411ca133626f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubstandardMember_740e9c66-3118-4c53-ab52-e0fda4991961" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubstandardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_3ef69331-670d-4a61-a450-ab3c1b79f043" xlink:to="loc_us-gaap_SubstandardMember_740e9c66-3118-4c53-ab52-e0fda4991961" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear_c3850ade-b5af-432d-85fb-b3c88f698003" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear_c3850ade-b5af-432d-85fb-b3c88f698003" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear_efe19405-0ad2-433b-8195-ddb66b8d7983" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear_efe19405-0ad2-433b-8195-ddb66b8d7983" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear_f832188a-68d8-41b6-9621-1a9a7e10f9ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear_f832188a-68d8-41b6-9621-1a9a7e10f9ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear_b7e91369-06cb-4910-b75a-32bb6bfd7d81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear_b7e91369-06cb-4910-b75a-32bb6bfd7d81" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear_f5342480-d998-4eb1-aaa6-650ab09189af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear_f5342480-d998-4eb1-aaa6-650ab09189af" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear_361de129-022b-4e03-bad5-46f9cb27b09d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear_361de129-022b-4e03-bad5-46f9cb27b09d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolving_abca3df4-ae55-4a1a-a1ff-d031a06645e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolving"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableRevolving_abca3df4-ae55-4a1a-a1ff-d031a06645e6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoan_9b6bcace-985b-4603-9d2f-216486185cfc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolvingConvertedToTermLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoan_9b6bcace-985b-4603-9d2f-216486185cfc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_5f67958b-6dc4-49e0-8315-4910821fac7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_2cfd43a7-36bc-4d86-b494-0f23064f6a75" xlink:to="loc_us-gaap_NotesReceivableGross_5f67958b-6dc4-49e0-8315-4910821fac7a" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofGrossChargeoffsbyOriginationYearandClassDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofGrossChargeoffsbyOriginationYearandClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofGrossChargeoffsbyOriginationYearandClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8acf-44be-7df3-8602-ead8d3f2942a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8acf-44be-7df3-8602-ead8d3f2942a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8ad1-6d40-7681-839d-bc192055755f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8acf-44be-7df3-8602-ead8d3f2942a" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8ad1-6d40-7681-839d-bc192055755f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-7681-839d-bc192055755f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8ad1-6d40-7681-839d-bc192055755f" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-7681-839d-bc192055755f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-76f7-8b95-569f7ab58993" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8ad1-6d40-7681-839d-bc192055755f" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-76f7-8b95-569f7ab58993" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019c8ad1-6d40-7b1c-9389-339e6a494be2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-76f7-8b95-569f7ab58993" xlink:to="loc_us-gaap_CommercialRealEstateMember_019c8ad1-6d40-7b1c-9389-339e6a494be2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019c8ad1-6d40-72cd-bbda-f67b19295332" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-76f7-8b95-569f7ab58993" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019c8ad1-6d40-72cd-bbda-f67b19295332" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_019c8ad1-6d40-715a-8a42-135dc3b0fb82" xlink:href="fsbc-20251231.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-76f7-8b95-569f7ab58993" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_019c8ad1-6d40-715a-8a42-135dc3b0fb82" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019c8ad1-6d40-79ae-941d-b4f1bd9c0ee8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8ad1-6d40-76f7-8b95-569f7ab58993" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019c8ad1-6d40-79ae-941d-b4f1bd9c0ee8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearOneOriginatedCurrentFiscalYearWriteoff_019c8ad1-6d40-74c1-b90a-dadc46cdef5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearOneOriginatedCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableYearOneOriginatedCurrentFiscalYearWriteoff_019c8ad1-6d40-74c1-b90a-dadc46cdef5b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-724d-b5a5-2410a2199ff1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-724d-b5a5-2410a2199ff1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7cbf-9cef-bd5a757bb8e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7cbf-9cef-bd5a757bb8e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7faf-bc9a-82c0fbb20bf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7faf-bc9a-82c0fbb20bf6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7530-942a-57523a1688b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7530-942a-57523a1688b4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7adf-8495-2edd2b615b30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019c8ad1-6d40-7adf-8495-2edd2b615b30" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolvingWriteoff_019c8ad1-6d40-7a4b-9d5b-6f483a9ef00f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolvingWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableRevolvingWriteoff_019c8ad1-6d40-7a4b-9d5b-6f483a9ef00f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoanWriteoff_019c8ad1-6d40-7006-8916-72c692873759" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolvingConvertedToTermLoanWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoanWriteoff_019c8ad1-6d40-7006-8916-72c692873759" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019c8ad1-6d40-7330-bd87-825cff2e5bb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8acf-44be-7e63-aa86-a7f3a5a00c6e" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019c8ad1-6d40-7330-bd87-825cff2e5bb7" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofAgeAnalysisofPastDueLoansbyClassDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofAgeAnalysisofPastDueLoansbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofAgeAnalysisofPastDueLoansbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_a12338e5-243b-4463-b624-400adb7fbca5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ce63b5a6-5e57-449e-8bd4-726aff80a76f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_a12338e5-243b-4463-b624-400adb7fbca5" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ce63b5a6-5e57-449e-8bd4-726aff80a76f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_bc421c70-7a71-4495-9a90-616f864ca1a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ce63b5a6-5e57-449e-8bd4-726aff80a76f" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_bc421c70-7a71-4495-9a90-616f864ca1a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_bc421c70-7a71-4495-9a90-616f864ca1a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_bc421c70-7a71-4495-9a90-616f864ca1a4" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_bc421c70-7a71-4495-9a90-616f864ca1a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_bc421c70-7a71-4495-9a90-616f864ca1a4" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_a6405e4b-3a2b-42be-9a96-3b2a3a26518a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_us-gaap_CommercialRealEstateMember_a6405e4b-3a2b-42be-9a96-3b2a3a26518a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_12cea60c-8a21-49c4-8691-63609cd68b84" xlink:href="fsbc-20251231.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_12cea60c-8a21-49c4-8691-63609cd68b84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_3ab52b44-e603-413d-b7a2-3355344369b8" xlink:href="fsbc-20251231.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_fsbc_CommercialConstructionLoansMember_3ab52b44-e603-413d-b7a2-3355344369b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_744a5b22-9635-47d7-b5d3-ffd199688fe4" xlink:href="fsbc-20251231.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_744a5b22-9635-47d7-b5d3-ffd199688fe4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_75031cb9-c336-479d-b626-60f5014b3e33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_us-gaap_ResidentialRealEstateMember_75031cb9-c336-479d-b626-60f5014b3e33" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_e83f6880-473c-4b64-a9ec-1bfe5949dd2c" xlink:href="fsbc-20251231.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_fsbc_FarmlandMember_e83f6880-473c-4b64-a9ec-1bfe5949dd2c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_5db1cf05-bbfe-4713-b284-20c809519aed" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_fsbc_SecuredCommercialLoansMember_5db1cf05-bbfe-4713-b284-20c809519aed" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_a749166d-f305-4817-89ef-c99dd54171a2" xlink:href="fsbc-20251231.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_a749166d-f305-4817-89ef-c99dd54171a2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_e0215fa0-56e6-447f-a34f-4fdbf7bfc06f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d05f7344-e6bf-4c33-95fc-80655c632ab7" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_e0215fa0-56e6-447f-a34f-4fdbf7bfc06f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_2ffe359f-cd78-4d28-96ec-2c71e28dbafe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ce63b5a6-5e57-449e-8bd4-726aff80a76f" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_2ffe359f-cd78-4d28-96ec-2c71e28dbafe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_2ffe359f-cd78-4d28-96ec-2c71e28dbafe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_2ffe359f-cd78-4d28-96ec-2c71e28dbafe" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_2ffe359f-cd78-4d28-96ec-2c71e28dbafe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_2ffe359f-cd78-4d28-96ec-2c71e28dbafe" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivables30To59DaysPastDueMember_68683098-5111-4bb7-a317-3a95440fb07c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivables30To59DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:to="loc_us-gaap_FinancingReceivables30To59DaysPastDueMember_68683098-5111-4bb7-a317-3a95440fb07c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivables60To89DaysPastDueMember_72a80539-da61-46f7-bab6-49e8b29d585b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivables60To89DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:to="loc_us-gaap_FinancingReceivables60To89DaysPastDueMember_72a80539-da61-46f7-bab6-49e8b29d585b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_efca60fa-889d-4946-9db5-d6b34524178c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:to="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_efca60fa-889d-4946-9db5-d6b34524178c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetPastDueMember_5f855fff-176b-4979-a571-e6acf128fa79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialAssetPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:to="loc_us-gaap_FinancialAssetPastDueMember_5f855fff-176b-4979-a571-e6acf128fa79" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetNotPastDueMember_76974495-3c46-4633-832e-c02009507397" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialAssetNotPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_9d373026-eb4a-4606-9884-931866263978" xlink:to="loc_us-gaap_FinancialAssetNotPastDueMember_76974495-3c46-4633-832e-c02009507397" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_596c0c76-e308-4cc6-bb3e-d7adc006155e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_a12338e5-243b-4463-b624-400adb7fbca5" xlink:to="loc_us-gaap_NotesReceivableGross_596c0c76-e308-4cc6-bb3e-d7adc006155e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofCollateralDependentLoansbyClassDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofCollateralDependentLoansbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofCollateralDependentLoansbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019c7a27-2d21-7853-981e-b043cf04ea51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019c7a27-2d21-77dc-b229-a9e02fb9c170" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019c7a27-2d21-7853-981e-b043cf04ea51" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019c7a27-2d21-77dc-b229-a9e02fb9c170" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c7a27-2d21-70c9-99cd-6fb288d761f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019c7a27-2d21-77dc-b229-a9e02fb9c170" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c7a27-2d21-70c9-99cd-6fb288d761f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c7a27-2d21-70c9-99cd-6fb288d761f0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c7a27-2d21-70c9-99cd-6fb288d761f0" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c7a27-2d21-70c9-99cd-6fb288d761f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c7a27-2d21-79c1-ac5a-76548114897e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c7a27-2d21-70c9-99cd-6fb288d761f0" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c7a27-2d21-79c1-ac5a-76548114897e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019c7a27-2d21-71c1-a212-de58027fcbc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c7a27-2d21-79c1-ac5a-76548114897e" xlink:to="loc_us-gaap_CommercialRealEstateMember_019c7a27-2d21-71c1-a212-de58027fcbc0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019c7a27-2d21-78c4-bacd-ed8baa65d5ba" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c7a27-2d21-79c1-ac5a-76548114897e" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019c7a27-2d21-78c4-bacd-ed8baa65d5ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019c7a27-2d21-75d8-9b24-f009bd26f56f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019c7a27-2d21-7853-981e-b043cf04ea51" xlink:to="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019c7a27-2d21-75d8-9b24-f009bd26f56f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofLoansModifiedduetoFinancialDistressDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofLoansModifiedduetoFinancialDistressDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofLoansModifiedduetoFinancialDistressDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8adc-0ad3-7760-95ec-5125f3a28325" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8adc-0ad3-78bb-8d55-bcbdb3bdd7b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8adc-0ad3-7760-95ec-5125f3a28325" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8adc-0ad3-78bb-8d55-bcbdb3bdd7b3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8adc-0ad3-77a7-a4a8-793f43a2e325" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8adc-0ad3-78bb-8d55-bcbdb3bdd7b3" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8adc-0ad3-77a7-a4a8-793f43a2e325" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8adc-0ad3-77a7-a4a8-793f43a2e325_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8adc-0ad3-77a7-a4a8-793f43a2e325" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8adc-0ad3-77a7-a4a8-793f43a2e325_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8adc-0ad3-7cf8-b979-2d747c05fe34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c8adc-0ad3-77a7-a4a8-793f43a2e325" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8adc-0ad3-7cf8-b979-2d747c05fe34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019c8adc-0ad3-7676-a6f1-04b5101ac7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c8adc-0ad3-7cf8-b979-2d747c05fe34" xlink:to="loc_us-gaap_CommercialRealEstateMember_019c8adc-0ad3-7676-a6f1-04b5101ac7e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationAxis_019c8ade-4fd8-7292-845a-bc5a7abb3abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoanRestructuringModificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019c8adc-0ad3-78bb-8d55-bcbdb3bdd7b3" xlink:to="loc_us-gaap_LoanRestructuringModificationAxis_019c8ade-4fd8-7292-845a-bc5a7abb3abc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationDomain_019c8ade-4fd8-7292-845a-bc5a7abb3abc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoanRestructuringModificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoanRestructuringModificationAxis_019c8ade-4fd8-7292-845a-bc5a7abb3abc" xlink:to="loc_us-gaap_LoanRestructuringModificationDomain_019c8ade-4fd8-7292-845a-bc5a7abb3abc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationDomain_019c8ade-4fd8-7dca-b05e-193ab25b3738" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoanRestructuringModificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoanRestructuringModificationAxis_019c8ade-4fd8-7292-845a-bc5a7abb3abc" xlink:to="loc_us-gaap_LoanRestructuringModificationDomain_019c8ade-4fd8-7dca-b05e-193ab25b3738" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentDeferralMember_019c8ade-79ac-716e-9aff-c6dde43d8b16" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentDeferralMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019c8ade-4fd8-7dca-b05e-193ab25b3738" xlink:to="loc_us-gaap_PaymentDeferralMember_019c8ade-79ac-716e-9aff-c6dde43d8b16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2_019c8adc-aa2c-7730-8593-f7704dd66df0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8adc-0ad3-7760-95ec-5125f3a28325" xlink:to="loc_us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2_019c8adc-aa2c-7730-8593-f7704dd66df0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent_019c8adc-aa2c-71b7-b147-23f009aa32c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019c8adc-0ad3-7760-95ec-5125f3a28325" xlink:to="loc_us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent_019c8adc-aa2c-71b7-b147-23f009aa32c6" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofFinancialEffectofLoansModifiedduetoFinancialDistressDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofFinancialEffectofLoansModifiedduetoFinancialDistressDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofFinancialEffectofLoansModifiedduetoFinancialDistressDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationsLineItems_019c9b25-8ce9-7736-89c7-646f28b7d244" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModificationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019c9b25-8ce9-7678-89a4-7d89ec84ec27" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019c9b25-8ce9-7736-89c7-646f28b7d244" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019c9b25-8ce9-7678-89a4-7d89ec84ec27" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c9b25-8ce9-7311-ba5d-7202068f4358" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019c9b25-8ce9-7678-89a4-7d89ec84ec27" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c9b25-8ce9-7311-ba5d-7202068f4358" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c9b25-8ce9-7311-ba5d-7202068f4358_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c9b25-8ce9-7311-ba5d-7202068f4358" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c9b25-8ce9-7311-ba5d-7202068f4358_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c9b25-8ce9-7e53-8c2f-f21c1e63a826" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019c9b25-8ce9-7311-ba5d-7202068f4358" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c9b25-8ce9-7e53-8c2f-f21c1e63a826" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019c9b25-8ce9-7893-9c11-f54f34925060" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019c9b25-8ce9-7e53-8c2f-f21c1e63a826" xlink:to="loc_us-gaap_CommercialRealEstateMember_019c9b25-8ce9-7893-9c11-f54f34925060" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancingReceivableModifiedInPeriodReducedLoanPaymentByBorrowerPeriod_019c9b29-a1a4-7a48-ab2f-38c9c5e06497" xlink:href="fsbc-20251231.xsd#fsbc_FinancingReceivableModifiedInPeriodReducedLoanPaymentByBorrowerPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019c9b25-8ce9-7736-89c7-646f28b7d244" xlink:to="loc_fsbc_FinancingReceivableModifiedInPeriodReducedLoanPaymentByBorrowerPeriod_019c9b29-a1a4-7a48-ab2f-38c9c5e06497" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModifiedPaymentDeferralPeriod_019c9b2b-be00-74e1-896f-6ad93b4dbced" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModifiedPaymentDeferralPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019c9b25-8ce9-7736-89c7-646f28b7d244" xlink:to="loc_us-gaap_FinancingReceivableModifiedPaymentDeferralPeriod_019c9b2b-be00-74e1-896f-6ad93b4dbced" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofNonaccrualLoansSegregatedbyClassDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofNonaccrualLoansSegregatedbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofNonaccrualLoansSegregatedbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_e2739fcd-4483-4123-84bf-1adbe0e4efdb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_3277007a-36b2-4e62-a3b9-9eb6fd8e5931" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_e2739fcd-4483-4123-84bf-1adbe0e4efdb" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_3277007a-36b2-4e62-a3b9-9eb6fd8e5931" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_81036fab-d5d6-409a-a82b-ae6bbcd8eb63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_3277007a-36b2-4e62-a3b9-9eb6fd8e5931" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_81036fab-d5d6-409a-a82b-ae6bbcd8eb63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_81036fab-d5d6-409a-a82b-ae6bbcd8eb63_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_81036fab-d5d6-409a-a82b-ae6bbcd8eb63" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_81036fab-d5d6-409a-a82b-ae6bbcd8eb63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_6184e8a7-416a-4fa4-bf61-7d7f6ad91ce7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_81036fab-d5d6-409a-a82b-ae6bbcd8eb63" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_6184e8a7-416a-4fa4-bf61-7d7f6ad91ce7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_97ae4e78-9a72-4338-836f-897ebce3d497" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_6184e8a7-416a-4fa4-bf61-7d7f6ad91ce7" xlink:to="loc_us-gaap_CommercialRealEstateMember_97ae4e78-9a72-4338-836f-897ebce3d497" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_31063291-cb36-4a86-962b-b3fef1f8570c" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_6184e8a7-416a-4fa4-bf61-7d7f6ad91ce7" xlink:to="loc_fsbc_SecuredCommercialLoansMember_31063291-cb36-4a86-962b-b3fef1f8570c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus_d34c6a0b-dfa7-46a9-8406-c54b94a4e5dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_e2739fcd-4483-4123-84bf-1adbe0e4efdb" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus_d34c6a0b-dfa7-46a9-8406-c54b94a4e5dc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofAllowanceforLoanLossesDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofAllowanceforLoanLossesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofAllowanceforLoanLossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_8ea040c0-2194-4186-beed-b2504999577b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_5388fd9c-de8a-4af8-8e96-09d815e07e42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_8ea040c0-2194-4186-beed-b2504999577b" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_5388fd9c-de8a-4af8-8e96-09d815e07e42" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_aacfa6e6-989e-4b36-8634-dd46b82bc2d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_5388fd9c-de8a-4af8-8e96-09d815e07e42" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_aacfa6e6-989e-4b36-8634-dd46b82bc2d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_aacfa6e6-989e-4b36-8634-dd46b82bc2d6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_aacfa6e6-989e-4b36-8634-dd46b82bc2d6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_aacfa6e6-989e-4b36-8634-dd46b82bc2d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_aacfa6e6-989e-4b36-8634-dd46b82bc2d6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_beb5cfb2-b6a0-4f83-a5dd-9f96e0c31afc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_us-gaap_CommercialRealEstateMember_beb5cfb2-b6a0-4f83-a5dd-9f96e0c31afc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_0bcbf74d-bddd-4e39-b802-a829df5ac7a8" xlink:href="fsbc-20251231.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_0bcbf74d-bddd-4e39-b802-a829df5ac7a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_dc002fcd-3c2d-452b-b9f7-e0a12bd03261" xlink:href="fsbc-20251231.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_fsbc_CommercialConstructionLoansMember_dc002fcd-3c2d-452b-b9f7-e0a12bd03261" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_f2745670-a3f9-47a0-9c17-300c8be1edf1" xlink:href="fsbc-20251231.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_f2745670-a3f9-47a0-9c17-300c8be1edf1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_973473c8-0c78-4b8f-9a4c-7d825dfa5c1b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_us-gaap_ResidentialRealEstateMember_973473c8-0c78-4b8f-9a4c-7d825dfa5c1b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_dd11d576-92e3-4d79-9281-d1f3a964e2b8" xlink:href="fsbc-20251231.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_fsbc_FarmlandMember_dd11d576-92e3-4d79-9281-d1f3a964e2b8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_53bb1443-655c-44a4-84ac-76fe49dff9a5" xlink:href="fsbc-20251231.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_fsbc_SecuredCommercialLoansMember_53bb1443-655c-44a4-84ac-76fe49dff9a5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_6fa5a10e-c26a-492b-b947-c636981785a0" xlink:href="fsbc-20251231.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_6fa5a10e-c26a-492b-b947-c636981785a0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_f659ad8b-263c-45de-abf0-97164c478a47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_f659ad8b-263c-45de-abf0-97164c478a47" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnallocatedFinancingReceivablesMember_df7d537b-73f2-4bfb-926c-2cfcfcadebb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnallocatedFinancingReceivablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_f476917f-84cf-46a5-b600-561a2bf1ecf6" xlink:to="loc_us-gaap_UnallocatedFinancingReceivablesMember_df7d537b-73f2-4bfb-926c-2cfcfcadebb6" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_e98a10b4-1e7f-4633-8cb2-70db0d80ef2b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_5388fd9c-de8a-4af8-8e96-09d815e07e42" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_e98a10b4-1e7f-4633-8cb2-70db0d80ef2b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e98a10b4-1e7f-4633-8cb2-70db0d80ef2b_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_e98a10b4-1e7f-4633-8cb2-70db0d80ef2b" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e98a10b4-1e7f-4633-8cb2-70db0d80ef2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_cb96e687-70a3-4340-abd0-79c085d820d9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_e98a10b4-1e7f-4633-8cb2-70db0d80ef2b" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_cb96e687-70a3-4340-abd0-79c085d820d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_558e7c7c-c8ec-4e1b-acd4-6d1cdd262638" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_cb96e687-70a3-4340-abd0-79c085d820d9" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_558e7c7c-c8ec-4e1b-acd4-6d1cdd262638" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_694b92db-9e78-40b5-8d23-2f02ea939259" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_8ea040c0-2194-4186-beed-b2504999577b" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_694b92db-9e78-40b5-8d23-2f02ea939259" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_2c497a8f-d047-4bef-a3de-2da3dd4ab022" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_694b92db-9e78-40b5-8d23-2f02ea939259" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_2c497a8f-d047-4bef-a3de-2da3dd4ab022" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_4efa4239-b0dc-4efc-ac47-b6d50d0806c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_694b92db-9e78-40b5-8d23-2f02ea939259" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_4efa4239-b0dc-4efc-ac47-b6d50d0806c0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery_ecc4b6c3-f515-4186-8812-891f408d64a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_694b92db-9e78-40b5-8d23-2f02ea939259" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery_ecc4b6c3-f515-4186-8812-891f408d64a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLossesExpensed_40d513a9-0ff4-41a3-b544-7babc5c6f3c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLossesExpensed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_694b92db-9e78-40b5-8d23-2f02ea939259" xlink:to="loc_us-gaap_ProvisionForLoanLossesExpensed_40d513a9-0ff4-41a3-b544-7babc5c6f3c0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_fbc09fd8-b5a4-47e7-9d59-4dc782a8fcc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofUnfundedLoanCommitmentReserveDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LoansandAllowanceforCreditLossesScheduleofUnfundedLoanCommitmentReserveDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesScheduleofUnfundedLoanCommitmentReserveDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_b6041445-92a5-4543-9702-2c4bf0e78114" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ded5588b-15c1-4817-9bb0-c425bbf4914d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_b6041445-92a5-4543-9702-2c4bf0e78114" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ded5588b-15c1-4817-9bb0-c425bbf4914d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_987b80af-970b-4313-9114-8ee25841bce1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ded5588b-15c1-4817-9bb0-c425bbf4914d" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_987b80af-970b-4313-9114-8ee25841bce1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_987b80af-970b-4313-9114-8ee25841bce1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_987b80af-970b-4313-9114-8ee25841bce1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_987b80af-970b-4313-9114-8ee25841bce1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_48a26cfa-e198-45a3-9e80-7562a35f70ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_987b80af-970b-4313-9114-8ee25841bce1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_48a26cfa-e198-45a3-9e80-7562a35f70ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedLoanCommitmentMember_b5c44d8d-61cf-44cd-9231-529c8d7102c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_48a26cfa-e198-45a3-9e80-7562a35f70ac" xlink:to="loc_us-gaap_UnfundedLoanCommitmentMember_b5c44d8d-61cf-44cd-9231-529c8d7102c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_67edc331-4f45-453e-9b48-18de8db17fc6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_ded5588b-15c1-4817-9bb0-c425bbf4914d" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_67edc331-4f45-453e-9b48-18de8db17fc6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_67edc331-4f45-453e-9b48-18de8db17fc6_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_67edc331-4f45-453e-9b48-18de8db17fc6" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_67edc331-4f45-453e-9b48-18de8db17fc6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_88c91d79-85cb-4995-bc48-523ce60b1c71" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_67edc331-4f45-453e-9b48-18de8db17fc6" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_88c91d79-85cb-4995-bc48-523ce60b1c71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_c2404755-5474-4931-8028-d0eddabdd772" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_88c91d79-85cb-4995-bc48-523ce60b1c71" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_c2404755-5474-4931-8028-d0eddabdd772" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_859dfea4-9039-48cb-b61b-ecd6a0347c60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_b6041445-92a5-4543-9702-2c4bf0e78114" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_859dfea4-9039-48cb-b61b-ecd6a0347c60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_8e24e9c0-f980-428b-b6af-fa94c5d939eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_859dfea4-9039-48cb-b61b-ecd6a0347c60" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_8e24e9c0-f980-428b-b6af-fa94c5d939eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLossesExpensed_7ac7d5ae-eb3f-4c9c-9669-285fe78c3e4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLossesExpensed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_859dfea4-9039-48cb-b61b-ecd6a0347c60" xlink:to="loc_us-gaap_ProvisionForLoanLossesExpensed_7ac7d5ae-eb3f-4c9c-9669-285fe78c3e4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_7ffbebae-f233-4cae-9292-eb7da9ec6332" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/PremisesandEquipmentNetScheduleofPremisesandEquipmentDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#PremisesandEquipmentNetScheduleofPremisesandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/PremisesandEquipmentNetScheduleofPremisesandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5e1b2606-7f1e-4b11-8a83-0fdf74a227c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_9eae9f4a-017e-4f74-acd4-91b5373b8310" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5e1b2606-7f1e-4b11-8a83-0fdf74a227c4" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_9eae9f4a-017e-4f74-acd4-91b5373b8310" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0a0cf35d-3feb-490d-b22a-44030ba130ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_9eae9f4a-017e-4f74-acd4-91b5373b8310" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0a0cf35d-3feb-490d-b22a-44030ba130ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_0a0cf35d-3feb-490d-b22a-44030ba130ca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0a0cf35d-3feb-490d-b22a-44030ba130ca" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_0a0cf35d-3feb-490d-b22a-44030ba130ca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_07cf7ff0-d1c8-443f-af97-8851967fd176" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0a0cf35d-3feb-490d-b22a-44030ba130ca" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_07cf7ff0-d1c8-443f-af97-8851967fd176" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FurnitureFixturesAndEquipmentMember_1b7f141f-ba70-4c26-84ac-f6db444d7f4b" xlink:href="fsbc-20251231.xsd#fsbc_FurnitureFixturesAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_07cf7ff0-d1c8-443f-af97-8851967fd176" xlink:to="loc_fsbc_FurnitureFixturesAndEquipmentMember_1b7f141f-ba70-4c26-84ac-f6db444d7f4b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TenantImprovementMember_d05d1623-be8c-49f8-bf73-60ee21d3f898" xlink:href="fsbc-20251231.xsd#fsbc_TenantImprovementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_07cf7ff0-d1c8-443f-af97-8851967fd176" xlink:to="loc_fsbc_TenantImprovementMember_d05d1623-be8c-49f8-bf73-60ee21d3f898" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AutomobilesMember_96e8ef86-61f6-4b9d-b4c2-83796ddb1c02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AutomobilesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_07cf7ff0-d1c8-443f-af97-8851967fd176" xlink:to="loc_us-gaap_AutomobilesMember_96e8ef86-61f6-4b9d-b4c2-83796ddb1c02" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_933aa9f7-274c-41c3-8221-65dc25d376f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5e1b2606-7f1e-4b11-8a83-0fdf74a227c4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_933aa9f7-274c-41c3-8221-65dc25d376f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_c0b26cd7-bc40-4549-bed7-d59631919101" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5e1b2606-7f1e-4b11-8a83-0fdf74a227c4" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_c0b26cd7-bc40-4549-bed7-d59631919101" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_50df62c2-ad5c-4b30-a587-364d5989fffd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5e1b2606-7f1e-4b11-8a83-0fdf74a227c4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_50df62c2-ad5c-4b30-a587-364d5989fffd" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/PremisesandEquipmentNetNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#PremisesandEquipmentNetNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/PremisesandEquipmentNetNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_bc9b577c-31e1-450c-8447-6f16e1f7c68d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_618c54c7-13f3-43a9-8e4e-e85c7ed4587c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_bc9b577c-31e1-450c-8447-6f16e1f7c68d" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_618c54c7-13f3-43a9-8e4e-e85c7ed4587c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9cbb9dcf-bf1b-42a1-bf2e-7d6b63ddbbce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_618c54c7-13f3-43a9-8e4e-e85c7ed4587c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9cbb9dcf-bf1b-42a1-bf2e-7d6b63ddbbce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9cbb9dcf-bf1b-42a1-bf2e-7d6b63ddbbce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9cbb9dcf-bf1b-42a1-bf2e-7d6b63ddbbce" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9cbb9dcf-bf1b-42a1-bf2e-7d6b63ddbbce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_231d8be5-5b2e-49ef-8bad-6740eebddd06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_9cbb9dcf-bf1b-42a1-bf2e-7d6b63ddbbce" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_231d8be5-5b2e-49ef-8bad-6740eebddd06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_OccupancyFurnitureFixturesAndEquipmentMember_52861561-eacb-41ee-ad17-c122bc9d6d66" xlink:href="fsbc-20251231.xsd#fsbc_OccupancyFurnitureFixturesAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_231d8be5-5b2e-49ef-8bad-6740eebddd06" xlink:to="loc_fsbc_OccupancyFurnitureFixturesAndEquipmentMember_52861561-eacb-41ee-ad17-c122bc9d6d66" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_1d004a83-99f2-4d2e-a697-30d5a8aefb54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_bc9b577c-31e1-450c-8447-6f16e1f7c68d" xlink:to="loc_us-gaap_DepreciationAndAmortization_1d004a83-99f2-4d2e-a697-30d5a8aefb54" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InterestBearingDepositsScheduleofCompositionofNetworkDepositsDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#InterestBearingDepositsScheduleofCompositionofNetworkDepositsDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InterestBearingDepositsScheduleofCompositionofNetworkDepositsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_InterestBearingDepostisLineItems_416b9cff-4436-4464-8b55-f73f914877b3" xlink:href="fsbc-20251231.xsd#fsbc_InterestBearingDepostisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_InterestBearingDepositsTable_086d1f2d-d106-4e3a-9778-8346991c2b59" xlink:href="fsbc-20251231.xsd#fsbc_InterestBearingDepositsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_fsbc_InterestBearingDepostisLineItems_416b9cff-4436-4464-8b55-f73f914877b3" xlink:to="loc_fsbc_InterestBearingDepositsTable_086d1f2d-d106-4e3a-9778-8346991c2b59" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_2405aa46-ff03-465e-9f1e-7ce3d7a65f64" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_fsbc_InterestBearingDepositsTable_086d1f2d-d106-4e3a-9778-8346991c2b59" xlink:to="loc_srt_ProductOrServiceAxis_2405aa46-ff03-465e-9f1e-7ce3d7a65f64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_2405aa46-ff03-465e-9f1e-7ce3d7a65f64_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_2405aa46-ff03-465e-9f1e-7ce3d7a65f64" xlink:to="loc_srt_ProductsAndServicesDomain_2405aa46-ff03-465e-9f1e-7ce3d7a65f64_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_36d6b433-c16b-4cf6-bbcd-e8f64127b06a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_2405aa46-ff03-465e-9f1e-7ce3d7a65f64" xlink:to="loc_srt_ProductsAndServicesDomain_36d6b433-c16b-4cf6-bbcd-e8f64127b06a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CDARSMember_8c892263-7134-4e09-99e2-76af605df7d4" xlink:href="fsbc-20251231.xsd#fsbc_CDARSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_36d6b433-c16b-4cf6-bbcd-e8f64127b06a" xlink:to="loc_fsbc_CDARSMember_8c892263-7134-4e09-99e2-76af605df7d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ICSMember_19e0654c-e577-4a84-8ec6-b4b26fd97ca0" xlink:href="fsbc-20251231.xsd#fsbc_ICSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_36d6b433-c16b-4cf6-bbcd-e8f64127b06a" xlink:to="loc_fsbc_ICSMember_19e0654c-e577-4a84-8ec6-b4b26fd97ca0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TotalNetworkDeposits_79b4108c-5932-4ad5-9fcb-6fa0244c0a49" xlink:href="fsbc-20251231.xsd#fsbc_TotalNetworkDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_InterestBearingDepostisLineItems_416b9cff-4436-4464-8b55-f73f914877b3" xlink:to="loc_fsbc_TotalNetworkDeposits_79b4108c-5932-4ad5-9fcb-6fa0244c0a49" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LongTermDebtandOtherBorrowingsNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LongTermDebtandOtherBorrowingsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LongTermDebtandOtherBorrowingsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_161cf8fd-2bfb-4a8f-8978-d2a789a3a576" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_DebtInstrumentTable_161cf8fd-2bfb-4a8f-8978-d2a789a3a576" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_5ea06c38-dc2b-427a-9ab9-e1bc127ac43b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_161cf8fd-2bfb-4a8f-8978-d2a789a3a576" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_5ea06c38-dc2b-427a-9ab9-e1bc127ac43b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_5ea06c38-dc2b-427a-9ab9-e1bc127ac43b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_5ea06c38-dc2b-427a-9ab9-e1bc127ac43b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_5ea06c38-dc2b-427a-9ab9-e1bc127ac43b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_f3a39d2b-a425-4fba-b578-2be4897b4ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_5ea06c38-dc2b-427a-9ab9-e1bc127ac43b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_f3a39d2b-a425-4fba-b578-2be4897b4ef0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_2f7ad963-cb42-431e-a3fa-036ae842a161" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_f3a39d2b-a425-4fba-b578-2be4897b4ef0" xlink:to="loc_us-gaap_SubordinatedDebtMember_2f7ad963-cb42-431e-a3fa-036ae842a161" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_7c39429f-9c24-407c-9865-eb6e3ee0dd94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_f3a39d2b-a425-4fba-b578-2be4897b4ef0" xlink:to="loc_us-gaap_LineOfCreditMember_7c39429f-9c24-407c-9865-eb6e3ee0dd94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_52a46d40-5235-4122-b33e-75058aadbb8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_161cf8fd-2bfb-4a8f-8978-d2a789a3a576" xlink:to="loc_us-gaap_DebtInstrumentAxis_52a46d40-5235-4122-b33e-75058aadbb8b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_52a46d40-5235-4122-b33e-75058aadbb8b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_52a46d40-5235-4122-b33e-75058aadbb8b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_52a46d40-5235-4122-b33e-75058aadbb8b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_09cb2016-d3fd-47e8-85bf-8f9ce1935b66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_52a46d40-5235-4122-b33e-75058aadbb8b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_09cb2016-d3fd-47e8-85bf-8f9ce1935b66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FixedToFloatingRateSubordinatedNotesIssuedAugust2022Member_7dd3abc1-596b-4af9-a0e7-e1725d6f57a7" xlink:href="fsbc-20251231.xsd#fsbc_FixedToFloatingRateSubordinatedNotesIssuedAugust2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_09cb2016-d3fd-47e8-85bf-8f9ce1935b66" xlink:to="loc_fsbc_FixedToFloatingRateSubordinatedNotesIssuedAugust2022Member_7dd3abc1-596b-4af9-a0e7-e1725d6f57a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_EntityControlledByExistingAndFormerBoardMembersAndAffiliatesMember_20da40f2-1123-426b-aa04-c9e962fa3169" xlink:href="fsbc-20251231.xsd#fsbc_EntityControlledByExistingAndFormerBoardMembersAndAffiliatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_09cb2016-d3fd-47e8-85bf-8f9ce1935b66" xlink:to="loc_fsbc_EntityControlledByExistingAndFormerBoardMembersAndAffiliatesMember_20da40f2-1123-426b-aa04-c9e962fa3169" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveDiscountWindowMember_38d0e4cc-02c2-4d41-94a9-a55391d73923" xlink:href="fsbc-20251231.xsd#fsbc_FederalReserveDiscountWindowMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_09cb2016-d3fd-47e8-85bf-8f9ce1935b66" xlink:to="loc_fsbc_FederalReserveDiscountWindowMember_38d0e4cc-02c2-4d41-94a9-a55391d73923" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FiveUnsecuredFederalFundsLineOfCreditMember_44088956-c2dc-4fdf-8904-61974cb5e1fc" xlink:href="fsbc-20251231.xsd#fsbc_FiveUnsecuredFederalFundsLineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_09cb2016-d3fd-47e8-85bf-8f9ce1935b66" xlink:to="loc_fsbc_FiveUnsecuredFederalFundsLineOfCreditMember_44088956-c2dc-4fdf-8904-61974cb5e1fc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_8bc64c6d-d12a-4ca5-a1f3-72e1f7395e18" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_161cf8fd-2bfb-4a8f-8978-d2a789a3a576" xlink:to="loc_srt_RangeAxis_8bc64c6d-d12a-4ca5-a1f3-72e1f7395e18" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_8bc64c6d-d12a-4ca5-a1f3-72e1f7395e18_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_8bc64c6d-d12a-4ca5-a1f3-72e1f7395e18" xlink:to="loc_srt_RangeMember_8bc64c6d-d12a-4ca5-a1f3-72e1f7395e18_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_55062e44-aef1-4ad7-99cb-0bc5b6744ff5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_8bc64c6d-d12a-4ca5-a1f3-72e1f7395e18" xlink:to="loc_srt_RangeMember_55062e44-aef1-4ad7-99cb-0bc5b6744ff5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_a96d4a0c-6dbf-4cda-a1f4-3d99b819325e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_55062e44-aef1-4ad7-99cb-0bc5b6744ff5" xlink:to="loc_srt_MinimumMember_a96d4a0c-6dbf-4cda-a1f4-3d99b819325e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_91ff773a-19a4-476c-93e4-4beaa1ae2999" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_55062e44-aef1-4ad7-99cb-0bc5b6744ff5" xlink:to="loc_srt_MaximumMember_91ff773a-19a4-476c-93e4-4beaa1ae2999" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebt_5806670a-1a9e-443d-bdc0-7175cbc4be6d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_SubordinatedDebt_5806670a-1a9e-443d-bdc0-7175cbc4be6d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_51614597-370a-4be4-943a-ad4ab12e046b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_51614597-370a-4be4-943a-ad4ab12e046b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedBorrowingInterestRate_b39ef1b1-2ded-4826-b29e-335680aeb92c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedBorrowingInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_SubordinatedBorrowingInterestRate_b39ef1b1-2ded-4826-b29e-335680aeb92c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_f0dfba57-4e0b-41b6-a215-5bf08954c9d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_f0dfba57-4e0b-41b6-a215-5bf08954c9d0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtInstrumentFloatingInterestRate_f87ece15-b4ed-42b9-8992-ce7688f3bf26" xlink:href="fsbc-20251231.xsd#fsbc_DebtInstrumentFloatingInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_fsbc_DebtInstrumentFloatingInterestRate_f87ece15-b4ed-42b9-8992-ce7688f3bf26" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_a28e6948-cd0c-4a28-9d70-265ea403f302" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_a28e6948-cd0c-4a28-9d70-265ea403f302" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedAmortizationDeferredFinanceCosts_6356151c-f9ca-44ab-af55-0d91d10de81d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedAmortizationDeferredFinanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_AccumulatedAmortizationDeferredFinanceCosts_6356151c-f9ca-44ab-af55-0d91d10de81d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ffb714ce-3fdd-4238-94ce-66948d9674fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ffb714ce-3fdd-4238-94ce-66948d9674fa" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_3ca81bf1-fe98-469e-820a-b49a67bb48c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_3ca81bf1-fe98-469e-820a-b49a67bb48c9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_4c684172-40b5-432e-9c6d-fddc91d75bfd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_us-gaap_LineOfCredit_4c684172-40b5-432e-9c6d-fddc91d75bfd" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_NumberOfUnsecuredFederalFunds_cc5bbdd5-835b-4e19-8871-0e43b304ef5c" xlink:href="fsbc-20251231.xsd#fsbc_NumberOfUnsecuredFederalFunds"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_fsbc_NumberOfUnsecuredFederalFunds_cc5bbdd5-835b-4e19-8871-0e43b304ef5c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_NumberOfCorrespondentBanks_d0d9c290-9248-40d4-92ad-70a25e1a16d5" xlink:href="fsbc-20251231.xsd#fsbc_NumberOfCorrespondentBanks"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_40269c15-7847-45e5-8e89-f44af764d593" xlink:to="loc_fsbc_NumberOfCorrespondentBanks_d0d9c290-9248-40d4-92ad-70a25e1a16d5" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LongTermDebtandOtherBorrowingsScheduleofBorrowingCapacityDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#LongTermDebtandOtherBorrowingsScheduleofBorrowingCapacityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LongTermDebtandOtherBorrowingsScheduleofBorrowingCapacityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ee0d3d43-5137-481d-ae10-c92a6ee61d7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_6e9749cc-8ab8-40b1-86f6-b51222e32a58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ee0d3d43-5137-481d-ae10-c92a6ee61d7b" xlink:to="loc_us-gaap_DebtInstrumentTable_6e9749cc-8ab8-40b1-86f6-b51222e32a58" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_97cb6ff6-72c9-4768-84e1-04b1e05433c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6e9749cc-8ab8-40b1-86f6-b51222e32a58" xlink:to="loc_us-gaap_DebtInstrumentAxis_97cb6ff6-72c9-4768-84e1-04b1e05433c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_97cb6ff6-72c9-4768-84e1-04b1e05433c5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_97cb6ff6-72c9-4768-84e1-04b1e05433c5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_97cb6ff6-72c9-4768-84e1-04b1e05433c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_e811b724-dbc8-4987-93b1-9e8bc80842a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_97cb6ff6-72c9-4768-84e1-04b1e05433c5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_e811b724-dbc8-4987-93b1-9e8bc80842a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FHLBAgreementMember_b413d7d3-e56b-4362-be72-4c9d8ec9795e" xlink:href="fsbc-20251231.xsd#fsbc_FHLBAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e811b724-dbc8-4987-93b1-9e8bc80842a0" xlink:to="loc_fsbc_FHLBAgreementMember_b413d7d3-e56b-4362-be72-4c9d8ec9795e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_LettersOfCreditPledgedAsSecurityForStateOfCaliforniaDepositsMember_8ea63654-8140-447d-96f5-593035e11027" xlink:href="fsbc-20251231.xsd#fsbc_LettersOfCreditPledgedAsSecurityForStateOfCaliforniaDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e811b724-dbc8-4987-93b1-9e8bc80842a0" xlink:to="loc_fsbc_LettersOfCreditPledgedAsSecurityForStateOfCaliforniaDepositsMember_8ea63654-8140-447d-96f5-593035e11027" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecureLocalAgencyDepositMember_c5fe14b6-2d62-4bae-97fe-a89c79875d7b" xlink:href="fsbc-20251231.xsd#fsbc_SecureLocalAgencyDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e811b724-dbc8-4987-93b1-9e8bc80842a0" xlink:to="loc_fsbc_SecureLocalAgencyDepositMember_c5fe14b6-2d62-4bae-97fe-a89c79875d7b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_6d7ac594-1969-4184-abc5-43b4767424c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6e9749cc-8ab8-40b1-86f6-b51222e32a58" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_6d7ac594-1969-4184-abc5-43b4767424c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_6d7ac594-1969-4184-abc5-43b4767424c9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_6d7ac594-1969-4184-abc5-43b4767424c9" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_6d7ac594-1969-4184-abc5-43b4767424c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_eea2b203-e2c4-4e05-b281-9c9cbb5f3269" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_6d7ac594-1969-4184-abc5-43b4767424c9" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_eea2b203-e2c4-4e05-b281-9c9cbb5f3269" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_acce2a7c-ec02-4c4f-aa7d-cc27bdcf2235" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_eea2b203-e2c4-4e05-b281-9c9cbb5f3269" xlink:to="loc_us-gaap_LineOfCreditMember_acce2a7c-ec02-4c4f-aa7d-cc27bdcf2235" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_f84a31ee-e248-4e05-9345-501adf0db935" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_eea2b203-e2c4-4e05-b281-9c9cbb5f3269" xlink:to="loc_us-gaap_LetterOfCreditMember_f84a31ee-e248-4e05-9345-501adf0db935" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_0b11ab0f-ee4f-4ae4-89f5-1c423cd95abf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ee0d3d43-5137-481d-ae10-c92a6ee61d7b" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_0b11ab0f-ee4f-4ae4-89f5-1c423cd95abf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_ed8e10ac-e054-4592-a179-edf7c948b453" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ee0d3d43-5137-481d-ae10-c92a6ee61d7b" xlink:to="loc_us-gaap_LineOfCredit_ed8e10ac-e054-4592-a179-edf7c948b453" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_66c6b297-dabb-4b3c-8896-79170cfa9b26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ee0d3d43-5137-481d-ae10-c92a6ee61d7b" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_66c6b297-dabb-4b3c-8896-79170cfa9b26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_d2d47b16-2894-4828-a6d5-863885caa627" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ee0d3d43-5137-481d-ae10-c92a6ee61d7b" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_d2d47b16-2894-4828-a6d5-863885caa627" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/A401kBenefitPlanDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#A401kBenefitPlanDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/A401kBenefitPlanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanTable_383f7ec5-36d2-448b-a18c-d2e9c5aa542d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:to="loc_us-gaap_DefinedContributionPlanTable_383f7ec5-36d2-448b-a18c-d2e9c5aa542d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_ba0bfb11-af32-41f9-b784-9b6b498c6420" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DefinedContributionPlanTable_383f7ec5-36d2-448b-a18c-d2e9c5aa542d" xlink:to="loc_srt_RangeAxis_ba0bfb11-af32-41f9-b784-9b6b498c6420" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_ba0bfb11-af32-41f9-b784-9b6b498c6420_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_ba0bfb11-af32-41f9-b784-9b6b498c6420" xlink:to="loc_srt_RangeMember_ba0bfb11-af32-41f9-b784-9b6b498c6420_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_010bb215-ba20-4f3e-8d1d-394275570611" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_ba0bfb11-af32-41f9-b784-9b6b498c6420" xlink:to="loc_srt_RangeMember_010bb215-ba20-4f3e-8d1d-394275570611" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_67a17c09-a63f-4746-ad1c-90c92324ac7f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_010bb215-ba20-4f3e-8d1d-394275570611" xlink:to="loc_srt_MaximumMember_67a17c09-a63f-4746-ad1c-90c92324ac7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_c38d0c33-6502-4da0-b2f7-a9801449ef78" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DefinedContributionPlanTable_383f7ec5-36d2-448b-a18c-d2e9c5aa542d" xlink:to="loc_us-gaap_RetirementPlanNameAxis_c38d0c33-6502-4da0-b2f7-a9801449ef78" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_c38d0c33-6502-4da0-b2f7-a9801449ef78_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_c38d0c33-6502-4da0-b2f7-a9801449ef78" xlink:to="loc_us-gaap_RetirementPlanNameDomain_c38d0c33-6502-4da0-b2f7-a9801449ef78_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_e12d57d0-53f7-470e-8756-4e5754fee866" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_c38d0c33-6502-4da0-b2f7-a9801449ef78" xlink:to="loc_us-gaap_RetirementPlanNameDomain_e12d57d0-53f7-470e-8756-4e5754fee866" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A3EmployeeContributionMember_c0d5f893-5ec4-4f75-8df3-70a148b75e51" xlink:href="fsbc-20251231.xsd#fsbc_A3EmployeeContributionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_e12d57d0-53f7-470e-8756-4e5754fee866" xlink:to="loc_fsbc_A3EmployeeContributionMember_c0d5f893-5ec4-4f75-8df3-70a148b75e51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Exceeding3EmployeeContributionMember_45fe8b0b-927c-4dbc-8653-0197a8c3f02b" xlink:href="fsbc-20251231.xsd#fsbc_Exceeding3EmployeeContributionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_e12d57d0-53f7-470e-8756-4e5754fee866" xlink:to="loc_fsbc_Exceeding3EmployeeContributionMember_45fe8b0b-927c-4dbc-8653-0197a8c3f02b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent_019c9adb-a9c1-7544-a27c-9b0e9b1b4627" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:to="loc_us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent_019c9adb-a9c1-7544-a27c-9b0e9b1b4627" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch_71f8a7ed-80ee-4342-87c3-c13e313e1c26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:to="loc_us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch_71f8a7ed-80ee-4342-87c3-c13e313e1c26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent_5d0b57be-a15d-4daa-8b9f-0f0cb0684b18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:to="loc_us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent_5d0b57be-a15d-4daa-8b9f-0f0cb0684b18" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage_714464ce-0932-41bb-90b6-7dd11dcbe07c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:to="loc_us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage_714464ce-0932-41bb-90b6-7dd11dcbe07c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount_4e862782-98ab-4b64-a3b4-b2fbbb3b5aa4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedContributionPlanDisclosureLineItems_12277fb2-f217-4325-8ed8-1f9c0fd59239" xlink:to="loc_us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount_4e862782-98ab-4b64-a3b4-b2fbbb3b5aa4" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/RelatedPartyTransactionsScheduleofLoanstoRelatedPartiesDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#RelatedPartyTransactionsScheduleofLoanstoRelatedPartiesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/RelatedPartyTransactionsScheduleofLoanstoRelatedPartiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_c8b5a154-b087-44a8-9090-abdfe334c208" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_110d9b0d-8ca3-4325-a2ce-4c95aaf9d111" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_c8b5a154-b087-44a8-9090-abdfe334c208" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_110d9b0d-8ca3-4325-a2ce-4c95aaf9d111" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_8e2841e4-c41c-4da3-bac4-ed236eaa0995" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_110d9b0d-8ca3-4325-a2ce-4c95aaf9d111" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_8e2841e4-c41c-4da3-bac4-ed236eaa0995" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_8e2841e4-c41c-4da3-bac4-ed236eaa0995_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_8e2841e4-c41c-4da3-bac4-ed236eaa0995" xlink:to="loc_us-gaap_RelatedPartyDomain_8e2841e4-c41c-4da3-bac4-ed236eaa0995_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_c8240623-b67e-4265-b52b-d9abfc89f3da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_8e2841e4-c41c-4da3-bac4-ed236eaa0995" xlink:to="loc_us-gaap_RelatedPartyDomain_c8240623-b67e-4265-b52b-d9abfc89f3da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_e8bef56d-2983-4cf8-961c-3f48fd486d89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_c8240623-b67e-4265-b52b-d9abfc89f3da" xlink:to="loc_us-gaap_RelatedPartyMember_e8bef56d-2983-4cf8-961c-3f48fd486d89" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_OtherReceivablesRollForward_098f8f6f-e752-49fb-bc67-c1db5d5e688e" xlink:href="fsbc-20251231.xsd#fsbc_OtherReceivablesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_c8b5a154-b087-44a8-9090-abdfe334c208" xlink:to="loc_fsbc_OtherReceivablesRollForward_098f8f6f-e752-49fb-bc67-c1db5d5e688e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivables_45dd5bfc-852f-4b1e-b223-950fdf36aaaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_OtherReceivablesRollForward_098f8f6f-e752-49fb-bc67-c1db5d5e688e" xlink:to="loc_us-gaap_OtherReceivables_45dd5bfc-852f-4b1e-b223-950fdf36aaaf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForAdvanceToAffiliate_04b0f96b-4f0d-4491-9434-45a721125642" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForAdvanceToAffiliate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_OtherReceivablesRollForward_098f8f6f-e752-49fb-bc67-c1db5d5e688e" xlink:to="loc_us-gaap_PaymentsForAdvanceToAffiliate_04b0f96b-4f0d-4491-9434-45a721125642" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromCollectionOfOtherReceivables_e8af86fd-108f-47be-8379-3d67ea6424bd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromCollectionOfOtherReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_OtherReceivablesRollForward_098f8f6f-e752-49fb-bc67-c1db5d5e688e" xlink:to="loc_us-gaap_ProceedsFromCollectionOfOtherReceivables_e8af86fd-108f-47be-8379-3d67ea6424bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivables_a0837c36-6065-4500-80e5-5df181f0fdcf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivables"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/EquityIncentivePlanNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#EquityIncentivePlanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/EquityIncentivePlanNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019b271c-b3d5-7dbf-985d-c0eaaa502828" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:to="loc_srt_RangeAxis_019b271c-b3d5-7dbf-985d-c0eaaa502828" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b271c-b3d5-7dbf-985d-c0eaaa502828_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019b271c-b3d5-7dbf-985d-c0eaaa502828" xlink:to="loc_srt_RangeMember_019b271c-b3d5-7dbf-985d-c0eaaa502828_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b271c-b3d5-72da-9cbb-e65bdd5e2ac7" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019b271c-b3d5-7dbf-985d-c0eaaa502828" xlink:to="loc_srt_RangeMember_019b271c-b3d5-72da-9cbb-e65bdd5e2ac7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019b271c-b3d5-770b-bbe0-98746f87ef37" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b271c-b3d5-72da-9cbb-e65bdd5e2ac7" xlink:to="loc_srt_MaximumMember_019b271c-b3d5-770b-bbe0-98746f87ef37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019b271c-b3d5-7b14-a473-288bf4ecdd1e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b271c-b3d5-72da-9cbb-e65bdd5e2ac7" xlink:to="loc_srt_MinimumMember_019b271c-b3d5-7b14-a473-288bf4ecdd1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_019b271c-b3d5-75b4-8902-7d5f55bf0806" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:to="loc_us-gaap_GranteeStatusAxis_019b271c-b3d5-75b4-8902-7d5f55bf0806" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75b4-8902-7d5f55bf0806_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_019b271c-b3d5-75b4-8902-7d5f55bf0806" xlink:to="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75b4-8902-7d5f55bf0806_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_019b271c-b3d5-75b4-8902-7d5f55bf0806" xlink:to="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementExecutivesAndDirectorsMember_019b271c-b3d5-7630-93be-64bddfc43828" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedPaymentArrangementExecutivesAndDirectorsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:to="loc_fsbc_ShareBasedPaymentArrangementExecutivesAndDirectorsMember_019b271c-b3d5-7630-93be-64bddfc43828" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019b271c-b3d5-7f60-b08a-fd06585b9600" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedPaymentArrangementEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019b271c-b3d5-7f60-b08a-fd06585b9600" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementExecutivesMember_019b271c-b3d5-71b7-9f4c-fbc6c1694c54" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedPaymentArrangementExecutivesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:to="loc_fsbc_ShareBasedPaymentArrangementExecutivesMember_019b271c-b3d5-71b7-9f4c-fbc6c1694c54" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementDirectorsMember_019b271c-b3d5-7c3d-b3e6-88989f85b1fd" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedPaymentArrangementDirectorsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:to="loc_fsbc_ShareBasedPaymentArrangementDirectorsMember_019b271c-b3d5-7c3d-b3e6-88989f85b1fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementNonemployeeMember_019b271f-1b81-7c4f-bc9f-e9373158cd84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedPaymentArrangementNonemployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019b271c-b3d5-75f6-bb8c-36d267d1c073" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementNonemployeeMember_019b271f-1b81-7c4f-bc9f-e9373158cd84" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019b271c-b3d5-7f9e-b387-3e9353b66061" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:to="loc_srt_TitleOfIndividualAxis_019b271c-b3d5-7f9e-b387-3e9353b66061" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b271c-b3d5-7f9e-b387-3e9353b66061_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019b271c-b3d5-7f9e-b387-3e9353b66061" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b271c-b3d5-7f9e-b387-3e9353b66061_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b271c-b3d5-73c7-8950-e4294441fbaa" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019b271c-b3d5-7f9e-b387-3e9353b66061" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b271c-b3d5-73c7-8950-e4294441fbaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OfficerMember_019b271c-b3d5-7286-8ded-ec6133733d35" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b271c-b3d5-73c7-8950-e4294441fbaa" xlink:to="loc_srt_OfficerMember_019b271c-b3d5-7286-8ded-ec6133733d35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019b271c-b3d5-7a81-82ea-d1d273580e01" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b271c-b3d5-73c7-8950-e4294441fbaa" xlink:to="loc_srt_ExecutiveOfficerMember_019b271c-b3d5-7a81-82ea-d1d273580e01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019b271c-b3d5-7a4f-af85-9c4fe39c5591" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:to="loc_us-gaap_PlanNameAxis_019b271c-b3d5-7a4f-af85-9c4fe39c5591" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019b271c-b3d5-7a4f-af85-9c4fe39c5591_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019b271c-b3d5-7a4f-af85-9c4fe39c5591" xlink:to="loc_us-gaap_PlanNameDomain_019b271c-b3d5-7a4f-af85-9c4fe39c5591_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019b271c-b3d5-7113-be84-12d55395f3b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019b271c-b3d5-7a4f-af85-9c4fe39c5591" xlink:to="loc_us-gaap_PlanNameDomain_019b271c-b3d5-7113-be84-12d55395f3b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_EquityIncentivePlanMember_019b271c-b3d5-70cd-9fec-3de2bf93fd92" xlink:href="fsbc-20251231.xsd#fsbc_EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019b271c-b3d5-7113-be84-12d55395f3b6" xlink:to="loc_fsbc_EquityIncentivePlanMember_019b271c-b3d5-70cd-9fec-3de2bf93fd92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b271f-1b81-7350-9500-ba23ea520659" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:to="loc_us-gaap_AwardTypeAxis_019b271f-1b81-7350-9500-ba23ea520659" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-7350-9500-ba23ea520659_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b271f-1b81-7350-9500-ba23ea520659" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-7350-9500-ba23ea520659_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-764f-8341-a2d1c69e25c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b271f-1b81-7350-9500-ba23ea520659" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-764f-8341-a2d1c69e25c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PerformanceBasedRestrictedStockUnitsPSUsMember_019b271f-1b81-7034-97b2-b1fd6b00d95b" xlink:href="fsbc-20251231.xsd#fsbc_PerformanceBasedRestrictedStockUnitsPSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-764f-8341-a2d1c69e25c6" xlink:to="loc_fsbc_PerformanceBasedRestrictedStockUnitsPSUsMember_019b271f-1b81-7034-97b2-b1fd6b00d95b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019b271f-1b81-7517-af5b-032345eeb223" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-764f-8341-a2d1c69e25c6" xlink:to="loc_us-gaap_RestrictedStockMember_019b271f-1b81-7517-af5b-032345eeb223" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019c7a39-45ad-7cc3-ae0a-52efb1a301d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b271f-1b81-764f-8341-a2d1c69e25c6" xlink:to="loc_us-gaap_PerformanceSharesMember_019c7a39-45ad-7cc3-ae0a-52efb1a301d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019c8ae5-6ac3-758b-96c9-08f615766d88" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationPerformancePercentilePositionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d6a176e8-603e-4985-8d58-dcada66019cb" xlink:to="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019c8ae5-6ac3-758b-96c9-08f615766d88" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-758b-96c9-08f615766d88_default" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationPercentilePositionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019c8ae5-6ac3-758b-96c9-08f615766d88" xlink:to="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-758b-96c9-08f615766d88_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-7ccf-8160-5bd912c888d3" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationPercentilePositionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019c8ae5-6ac3-758b-96c9-08f615766d88" xlink:to="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-7ccf-8160-5bd912c888d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A60thPercentileMember_019c8ae5-c465-79d9-bb61-65fdbd764aac" xlink:href="fsbc-20251231.xsd#fsbc_A60thPercentileMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-7ccf-8160-5bd912c888d3" xlink:to="loc_fsbc_A60thPercentileMember_019c8ae5-c465-79d9-bb61-65fdbd764aac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A70thPercentileMember_019c8ae5-fa87-74d2-91c6-f0b0115e78d0" xlink:href="fsbc-20251231.xsd#fsbc_A70thPercentileMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-7ccf-8160-5bd912c888d3" xlink:to="loc_fsbc_A70thPercentileMember_019c8ae5-fa87-74d2-91c6-f0b0115e78d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A80thPercentileOrHigherMember_019c8ae6-3b7e-7641-828c-cb5ffc2dbf79" xlink:href="fsbc-20251231.xsd#fsbc_A80thPercentileOrHigherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_ShareBasedCompensationPercentilePositionDomain_019c8ae5-6ac3-7ccf-8160-5bd912c888d3" xlink:to="loc_fsbc_A80thPercentileOrHigherMember_019c8ae6-3b7e-7641-828c-cb5ffc2dbf79" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019b271c-b3d5-7252-9da6-8461d1c9af31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019b271c-b3d5-7252-9da6-8461d1c9af31" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019b271c-b3d5-7559-afe2-6fbc890b4b0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019b271c-b3d5-7559-afe2-6fbc890b4b0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019b271c-b3d5-7170-8a8a-6288db826fb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019b271c-b3d5-7170-8a8a-6288db826fb2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsAwardPayoutsPercentage_019b271f-1b81-7ad0-9ad8-8e4eb4689172" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsAwardPayoutsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsAwardPayoutsPercentage_019b271f-1b81-7ad0-9ad8-8e4eb4689172" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019b271f-1b81-7ce6-945c-1285a3a97221" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensation_019b271f-1b81-7ce6-945c-1285a3a97221" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019b271f-1b81-7bf5-9bf8-9f2aa08f8512" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019b271f-1b81-7bf5-9bf8-9f2aa08f8512" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019b271f-1b81-7378-ad26-b5ea00d19935" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019b271f-1b81-7378-ad26-b5ea00d19935" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019b59e8-e7aa-758e-9ceb-177ce8f38051" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019b59e8-e7aa-758e-9ceb-177ce8f38051" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019b59e6-4318-74c9-8537-86756c5dbda5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019b59e6-4318-74c9-8537-86756c5dbda5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetAwardGrantPercentage_019c91c6-3ea5-70d4-9f8c-3ed0a4b41a48" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetAwardGrantPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetAwardGrantPercentage_019c91c6-3ea5-70d4-9f8c-3ed0a4b41a48" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetAwardNumberOfSharesExpectedToVest_019c91cf-80f4-7434-a7b2-9efcd3a55547" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetAwardNumberOfSharesExpectedToVest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetAwardNumberOfSharesExpectedToVest_019c91cf-80f4-7434-a7b2-9efcd3a55547" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementNonvestedAwardExcludingOptionCostNotYetRecognizedAmountExpected_019c91d5-cf1b-7fc1-9293-130987b2a2c1" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedPaymentArrangementNonvestedAwardExcludingOptionCostNotYetRecognizedAmountExpected"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_fsbc_ShareBasedPaymentArrangementNonvestedAwardExcludingOptionCostNotYetRecognizedAmountExpected_019c91d5-cf1b-7fc1-9293-130987b2a2c1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedWeightedAverageRemainingTerm_019b271f-1b81-7431-a250-e5eb1247d019" xlink:href="fsbc-20251231.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedWeightedAverageRemainingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_25ab5a7b-d3ba-40a4-8458-69f79a256415" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedWeightedAverageRemainingTerm_019b271f-1b81-7431-a250-e5eb1247d019" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/EquityIncentivePlanScheduleofUnvestedRestrictedSharesDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#EquityIncentivePlanScheduleofUnvestedRestrictedSharesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/EquityIncentivePlanScheduleofUnvestedRestrictedSharesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d7a89b45-fe3a-4b68-a7ef-98f87eba1e3a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0b476497-4467-428e-a1b7-4656de02e839" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d7a89b45-fe3a-4b68-a7ef-98f87eba1e3a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0b476497-4467-428e-a1b7-4656de02e839" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_1d8d7b8a-9748-41a2-b789-c411bf0eed8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0b476497-4467-428e-a1b7-4656de02e839" xlink:to="loc_us-gaap_AwardTypeAxis_1d8d7b8a-9748-41a2-b789-c411bf0eed8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d8d7b8a-9748-41a2-b789-c411bf0eed8d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_1d8d7b8a-9748-41a2-b789-c411bf0eed8d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d8d7b8a-9748-41a2-b789-c411bf0eed8d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0c1c26b2-c08d-49c7-85e6-b853ab05ed1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_1d8d7b8a-9748-41a2-b789-c411bf0eed8d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0c1c26b2-c08d-49c7-85e6-b853ab05ed1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_a51c8fb9-4884-4b3d-a032-b0a93d0f8978" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0c1c26b2-c08d-49c7-85e6-b853ab05ed1e" xlink:to="loc_us-gaap_RestrictedStockMember_a51c8fb9-4884-4b3d-a032-b0a93d0f8978" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0aefc262-80ab-4f30-8d18-b2f900d8d9f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d7a89b45-fe3a-4b68-a7ef-98f87eba1e3a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0aefc262-80ab-4f30-8d18-b2f900d8d9f4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_2157f177-a1aa-40c4-80ea-8cdbae6d7819" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0aefc262-80ab-4f30-8d18-b2f900d8d9f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_2157f177-a1aa-40c4-80ea-8cdbae6d7819" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_5572c7bb-d7e4-4713-a0e6-32a2d7fe283f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0aefc262-80ab-4f30-8d18-b2f900d8d9f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_5572c7bb-d7e4-4713-a0e6-32a2d7fe283f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_df61e325-fac1-47f4-883b-c8bea72480ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0aefc262-80ab-4f30-8d18-b2f900d8d9f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_df61e325-fac1-47f4-883b-c8bea72480ef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_20d7978a-608d-4cd9-84ec-4f2ccc959601" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0aefc262-80ab-4f30-8d18-b2f900d8d9f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_20d7978a-608d-4cd9-84ec-4f2ccc959601" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_c727f5e5-fa42-4caa-913b-7836241a79ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_c9dc82a9-eeae-44dd-b13c-24e281ddc9c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d7a89b45-fe3a-4b68-a7ef-98f87eba1e3a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_c9dc82a9-eeae-44dd-b13c-24e281ddc9c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_5e0b8059-8688-445a-a543-e0ed570d0169" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_c9dc82a9-eeae-44dd-b13c-24e281ddc9c0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_5e0b8059-8688-445a-a543-e0ed570d0169" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_04339ad6-a57d-4e74-a4cb-40279b977fae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_c9dc82a9-eeae-44dd-b13c-24e281ddc9c0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_04339ad6-a57d-4e74-a4cb-40279b977fae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_66938326-3285-47b5-92ba-497a083dbf55" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_c9dc82a9-eeae-44dd-b13c-24e281ddc9c0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_66938326-3285-47b5-92ba-497a083dbf55" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_5689b570-b3b1-4261-a00b-f048e0edaaa3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_c9dc82a9-eeae-44dd-b13c-24e281ddc9c0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_5689b570-b3b1-4261-a00b-f048e0edaaa3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_4c1de628-3724-4aa5-97c0-1e54e5b0a953" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/EquityIncentivePlanScheduleofUnvestedPerformanceShareActivityDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#EquityIncentivePlanScheduleofUnvestedPerformanceShareActivityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/EquityIncentivePlanScheduleofUnvestedPerformanceShareActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c7a32-0d66-7864-9c9f-2c3fcdbfa320" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c7a32-0d66-7197-a3d6-c10686d15452" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c7a32-0d66-7864-9c9f-2c3fcdbfa320" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c7a32-0d66-7197-a3d6-c10686d15452" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c7a32-0d66-74bc-b059-6c9a66f476e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c7a32-0d66-7197-a3d6-c10686d15452" xlink:to="loc_us-gaap_AwardTypeAxis_019c7a32-0d66-74bc-b059-6c9a66f476e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c7a32-0d66-74bc-b059-6c9a66f476e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c7a32-0d66-74bc-b059-6c9a66f476e6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c7a32-0d66-74bc-b059-6c9a66f476e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c7a32-0d66-77b0-ae4d-f5dfbd0a4afd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c7a32-0d66-74bc-b059-6c9a66f476e6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c7a32-0d66-77b0-ae4d-f5dfbd0a4afd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019c7a32-8b10-7095-b18c-7425c9dae708" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c7a32-0d66-77b0-ae4d-f5dfbd0a4afd" xlink:to="loc_us-gaap_PerformanceSharesMember_019c7a32-8b10-7095-b18c-7425c9dae708" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c7a32-0d66-7019-abd5-410e6e245319" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c7a32-0d66-7864-9c9f-2c3fcdbfa320" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c7a32-0d66-7019-abd5-410e6e245319" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019c7a32-0d66-7123-9153-9b524fd0f8e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c7a32-0d66-7019-abd5-410e6e245319" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019c7a32-0d66-7123-9153-9b524fd0f8e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019c7a32-0d66-77ac-bb5b-843a1f3b8161" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c7a32-0d66-7019-abd5-410e6e245319" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019c7a32-0d66-77ac-bb5b-843a1f3b8161" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019c7a32-0d66-73b4-9df6-78aba9a8bf45" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c7a32-0d66-7c52-bedf-805c21f2e648" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c7a32-0d66-7864-9c9f-2c3fcdbfa320" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c7a32-0d66-7c52-bedf-805c21f2e648" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019c7a32-0d66-7ed9-b2b4-467eea9af851" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c7a32-0d66-7c52-bedf-805c21f2e648" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019c7a32-0d66-7ed9-b2b4-467eea9af851" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c7a32-0d66-7750-8a65-b8b786b61505" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c7a32-0d66-7c52-bedf-805c21f2e648" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c7a32-0d66-7750-8a65-b8b786b61505" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019c7a32-0d66-7a44-b4c7-fc4ea2e37a62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CommitmentsandContingenciesScheduleofOtherCommitmentsDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#CommitmentsandContingenciesScheduleofOtherCommitmentsDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CommitmentsandContingenciesScheduleofOtherCommitmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_121db35a-75e9-43c4-88a1-1cc60ddb57bd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_c83ca393-0518-43db-a28a-1659d8ad1b2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_121db35a-75e9-43c4-88a1-1cc60ddb57bd" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_c83ca393-0518-43db-a28a-1659d8ad1b2c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsAxis_e4974040-06b5-437e-a980-a7489079928f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_c83ca393-0518-43db-a28a-1659d8ad1b2c" xlink:to="loc_us-gaap_OtherCommitmentsAxis_e4974040-06b5-437e-a980-a7489079928f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_e4974040-06b5-437e-a980-a7489079928f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherCommitmentsAxis_e4974040-06b5-437e-a980-a7489079928f" xlink:to="loc_us-gaap_OtherCommitmentsDomain_e4974040-06b5-437e-a980-a7489079928f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherCommitmentsAxis_e4974040-06b5-437e-a980-a7489079928f" xlink:to="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialLinesOfCreditMember_f7d61ecb-ab82-4250-b862-d2e3b3615cd7" xlink:href="fsbc-20251231.xsd#fsbc_CommercialLinesOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_CommercialLinesOfCreditMember_f7d61ecb-ab82-4250-b862-d2e3b3615cd7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedCommercialRealEstateLoansMember_df2079a1-0e05-407c-8697-9145f1aadc0f" xlink:href="fsbc-20251231.xsd#fsbc_UndisbursedCommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_UndisbursedCommercialRealEstateLoansMember_df2079a1-0e05-407c-8697-9145f1aadc0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedConstructionLoansMember_61e0f736-d3b8-40cb-9542-cca06e00a0a7" xlink:href="fsbc-20251231.xsd#fsbc_UndisbursedConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_UndisbursedConstructionLoansMember_61e0f736-d3b8-40cb-9542-cca06e00a0a7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_AgriculturalLinesOfCreditMember_3a8d6e5f-a876-4c1b-8e53-709da3a28646" xlink:href="fsbc-20251231.xsd#fsbc_AgriculturalLinesOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_AgriculturalLinesOfCreditMember_3a8d6e5f-a876-4c1b-8e53-709da3a28646" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedResidentialRealEstateLoansMember_544a21f0-6505-4d15-ba61-e9742d8038bb" xlink:href="fsbc-20251231.xsd#fsbc_UndisbursedResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_UndisbursedResidentialRealEstateLoansMember_544a21f0-6505-4d15-ba61-e9742d8038bb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedAgriculturalRealEstateLoansMember_b475867f-67a8-49bf-a279-9ebb80399e21" xlink:href="fsbc-20251231.xsd#fsbc_UndisbursedAgriculturalRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_UndisbursedAgriculturalRealEstateLoansMember_b475867f-67a8-49bf-a279-9ebb80399e21" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_OtherCommitmentsMember_019be9f6-500a-76f0-8945-c6474469f198" xlink:href="fsbc-20251231.xsd#fsbc_OtherCommitmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_399c7dab-1cd5-49df-a0ad-219e54adfcdc" xlink:to="loc_fsbc_OtherCommitmentsMember_019be9f6-500a-76f0-8945-c6474469f198" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitment_2def7046-d594-4857-a7e3-cd932376221b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_121db35a-75e9-43c4-88a1-1cc60ddb57bd" xlink:to="loc_us-gaap_OtherCommitment_2def7046-d594-4857-a7e3-cd932376221b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_dfc1559a-ab3a-4a5d-b725-deecce0d67fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_dfc1559a-ab3a-4a5d-b725-deecce0d67fb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_79a6c981-ba8e-4ef0-99a8-27f107ade6b3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_dfc1559a-ab3a-4a5d-b725-deecce0d67fb" xlink:to="loc_srt_RangeAxis_79a6c981-ba8e-4ef0-99a8-27f107ade6b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_79a6c981-ba8e-4ef0-99a8-27f107ade6b3_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_79a6c981-ba8e-4ef0-99a8-27f107ade6b3" xlink:to="loc_srt_RangeMember_79a6c981-ba8e-4ef0-99a8-27f107ade6b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a735ba5d-88c8-4fbf-a2a8-900c30e7b5a8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_79a6c981-ba8e-4ef0-99a8-27f107ade6b3" xlink:to="loc_srt_RangeMember_a735ba5d-88c8-4fbf-a2a8-900c30e7b5a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_8a806f93-137b-4bfc-a4cf-0727ee9b34af" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a735ba5d-88c8-4fbf-a2a8-900c30e7b5a8" xlink:to="loc_srt_MinimumMember_8a806f93-137b-4bfc-a4cf-0727ee9b34af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_6e28a2be-abb6-46ce-bb37-afa2c682655e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a735ba5d-88c8-4fbf-a2a8-900c30e7b5a8" xlink:to="loc_srt_MaximumMember_6e28a2be-abb6-46ce-bb37-afa2c682655e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_6679fa17-b2f2-4feb-90e8-c69133784d0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_dfc1559a-ab3a-4a5d-b725-deecce0d67fb" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_6679fa17-b2f2-4feb-90e8-c69133784d0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_6679fa17-b2f2-4feb-90e8-c69133784d0c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_6679fa17-b2f2-4feb-90e8-c69133784d0c" xlink:to="loc_us-gaap_RelatedPartyDomain_6679fa17-b2f2-4feb-90e8-c69133784d0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_2d313948-37c9-4e55-8ca5-7b251f2c1535" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_6679fa17-b2f2-4feb-90e8-c69133784d0c" xlink:to="loc_us-gaap_RelatedPartyDomain_2d313948-37c9-4e55-8ca5-7b251f2c1535" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AffiliatedEntityMember_7f9ca25c-2f64-4517-aba5-de9ad5bfecce" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_AffiliatedEntityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_2d313948-37c9-4e55-8ca5-7b251f2c1535" xlink:to="loc_srt_AffiliatedEntityMember_7f9ca25c-2f64-4517-aba5-de9ad5bfecce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_da4ef3ac-95cc-48e1-b845-123aa80889f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_dfc1559a-ab3a-4a5d-b725-deecce0d67fb" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_da4ef3ac-95cc-48e1-b845-123aa80889f7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_da4ef3ac-95cc-48e1-b845-123aa80889f7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_da4ef3ac-95cc-48e1-b845-123aa80889f7" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_da4ef3ac-95cc-48e1-b845-123aa80889f7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d7d32b24-e68d-4511-af1b-3dc5d1ad1352" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_da4ef3ac-95cc-48e1-b845-123aa80889f7" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d7d32b24-e68d-4511-af1b-3dc5d1ad1352" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedLoanCommitmentMember_e3289f4f-5dac-42b1-8446-e29c9a7e7b12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_d7d32b24-e68d-4511-af1b-3dc5d1ad1352" xlink:to="loc_us-gaap_UnfundedLoanCommitmentMember_e3289f4f-5dac-42b1-8446-e29c9a7e7b12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermAxis_92d7a96a-4ce9-4d9d-b3fe-f8290cbc8069" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseContractualTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_dfc1559a-ab3a-4a5d-b725-deecce0d67fb" xlink:to="loc_us-gaap_LeaseContractualTermAxis_92d7a96a-4ce9-4d9d-b3fe-f8290cbc8069" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_92d7a96a-4ce9-4d9d-b3fe-f8290cbc8069_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseContractualTermAxis_92d7a96a-4ce9-4d9d-b3fe-f8290cbc8069" xlink:to="loc_us-gaap_LeaseContractualTermDomain_92d7a96a-4ce9-4d9d-b3fe-f8290cbc8069_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_b9a2c0c4-ae47-4fdf-a1fe-0b67cab8404e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseContractualTermAxis_92d7a96a-4ce9-4d9d-b3fe-f8290cbc8069" xlink:to="loc_us-gaap_LeaseContractualTermDomain_b9a2c0c4-ae47-4fdf-a1fe-0b67cab8404e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SacramentoAdministrativeOfficeLeaseMaturityMay2026Member_fd67d4b2-85b1-45cc-a97e-685702bdfa37" xlink:href="fsbc-20251231.xsd#fsbc_SacramentoAdministrativeOfficeLeaseMaturityMay2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_b9a2c0c4-ae47-4fdf-a1fe-0b67cab8404e" xlink:to="loc_fsbc_SacramentoAdministrativeOfficeLeaseMaturityMay2026Member_fd67d4b2-85b1-45cc-a97e-685702bdfa37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TemporaryAdministrativeOfficeInSanFranciscoMember_aec495eb-b7da-4ac4-aa3e-2eee55af47e1" xlink:href="fsbc-20251231.xsd#fsbc_TemporaryAdministrativeOfficeInSanFranciscoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_b9a2c0c4-ae47-4fdf-a1fe-0b67cab8404e" xlink:to="loc_fsbc_TemporaryAdministrativeOfficeInSanFranciscoMember_aec495eb-b7da-4ac4-aa3e-2eee55af47e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_d47531c5-d3ec-4035-92a2-4d1ca163605e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_d47531c5-d3ec-4035-92a2-4d1ca163605e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateRelatedLoansPercentage_09cdc0d0-b54e-4911-a18b-2b5b62a3b080" xlink:href="fsbc-20251231.xsd#fsbc_RealEstateRelatedLoansPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_RealEstateRelatedLoansPercentage_09cdc0d0-b54e-4911-a18b-2b5b62a3b080" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Deposits5MillionOrMoreNumberOfDepositRelationships_59db48a5-d340-49c9-994b-0694516c37e8" xlink:href="fsbc-20251231.xsd#fsbc_Deposits5MillionOrMoreNumberOfDepositRelationships"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_Deposits5MillionOrMoreNumberOfDepositRelationships_59db48a5-d340-49c9-994b-0694516c37e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Deposits5MillionOrMorePerDeposit_ef8f2aaa-d769-41c1-9bcb-a00e524697a0" xlink:href="fsbc-20251231.xsd#fsbc_Deposits5MillionOrMorePerDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_Deposits5MillionOrMorePerDeposit_ef8f2aaa-d769-41c1-9bcb-a00e524697a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Deposits5MillionOrMoreAmount_a621d4d2-6404-4997-b29a-c3a5d8bc337e" xlink:href="fsbc-20251231.xsd#fsbc_Deposits5MillionOrMoreAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_Deposits5MillionOrMoreAmount_a621d4d2-6404-4997-b29a-c3a5d8bc337e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PercentageOfDepositsOver5MillionToDeposits_8db5b185-92a7-47af-bd89-de63393debaf" xlink:href="fsbc-20251231.xsd#fsbc_PercentageOfDepositsOver5MillionToDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_PercentageOfDepositsOver5MillionToDeposits_8db5b185-92a7-47af-bd89-de63393debaf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_LargestSingleDeposit_72b38407-b351-48f8-b600-f3bb5fe2b027" xlink:href="fsbc-20251231.xsd#fsbc_LargestSingleDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_LargestSingleDeposit_72b38407-b351-48f8-b600-f3bb5fe2b027" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PercentageOfLargestSingleDepositToDeposits_75336170-fc11-4876-af3d-3099761a0a2e" xlink:href="fsbc-20251231.xsd#fsbc_PercentageOfLargestSingleDepositToDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_PercentageOfLargestSingleDepositToDeposits_75336170-fc11-4876-af3d-3099761a0a2e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashUninsuredAmount_9c47c50d-a56e-4025-82fc-2687541bc5b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashUninsuredAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_CashUninsuredAmount_9c47c50d-a56e-4025-82fc-2687541bc5b5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019b2755-e2f3-7468-b4d3-2f8cf43a6689" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_EquityMethodInvestments_019b2755-e2f3-7468-b4d3-2f8cf43a6689" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_EquityMethodInvestmentRemainingCommitmentAmount_76f00e1f-0df6-4467-a30b-ae7ccbe0e372" xlink:href="fsbc-20251231.xsd#fsbc_EquityMethodInvestmentRemainingCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_fsbc_EquityMethodInvestmentRemainingCommitmentAmount_76f00e1f-0df6-4467-a30b-ae7ccbe0e372" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_892601bb-e90a-4df4-9fb8-d3971c72dfee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_892601bb-e90a-4df4-9fb8-d3971c72dfee" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_5fc30500-a648-4891-b733-268569ede331" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_OperatingLeaseExpense_5fc30500-a648-4891-b733-268569ede331" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_c7f525ce-0071-4e82-9c51-4ec49c8a2c12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_c7f525ce-0071-4e82-9c51-4ec49c8a2c12" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_7f9dfe4b-b94b-44e9-b76c-705b16a73d1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_8d1e48e9-2e57-48a2-8f29-2f87c9add5c9" xlink:to="loc_us-gaap_OperatingLeaseLiability_7f9dfe4b-b94b-44e9-b76c-705b16a73d1c" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CapitalRequirementsandRestrictionsonRetainedEarningsNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#CapitalRequirementsandRestrictionsonRetainedEarningsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CapitalRequirementsandRestrictionsonRetainedEarningsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_aee20af4-0f5e-44cc-9c09-8125b0204985" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_aee20af4-0f5e-44cc-9c09-8125b0204985" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_7a5bd782-5634-4759-9221-3c841a431c25" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_aee20af4-0f5e-44cc-9c09-8125b0204985" xlink:to="loc_srt_RangeAxis_7a5bd782-5634-4759-9221-3c841a431c25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7a5bd782-5634-4759-9221-3c841a431c25_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_7a5bd782-5634-4759-9221-3c841a431c25" xlink:to="loc_srt_RangeMember_7a5bd782-5634-4759-9221-3c841a431c25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_42cfd64c-fc54-4faf-b2d3-1890cfe3a07c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_7a5bd782-5634-4759-9221-3c841a431c25" xlink:to="loc_srt_RangeMember_42cfd64c-fc54-4faf-b2d3-1890cfe3a07c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_cc3af3b6-bf87-4517-ae8f-cf5464e5fe3d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_42cfd64c-fc54-4faf-b2d3-1890cfe3a07c" xlink:to="loc_srt_MinimumMember_cc3af3b6-bf87-4517-ae8f-cf5464e5fe3d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_095e966b-179f-4b75-a0d7-263b2dcb5d00" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_aee20af4-0f5e-44cc-9c09-8125b0204985" xlink:to="loc_srt_ConsolidatedEntitiesAxis_095e966b-179f-4b75-a0d7-263b2dcb5d00" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_095e966b-179f-4b75-a0d7-263b2dcb5d00_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_095e966b-179f-4b75-a0d7-263b2dcb5d00" xlink:to="loc_srt_ConsolidatedEntitiesDomain_095e966b-179f-4b75-a0d7-263b2dcb5d00_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_b8f79fa7-76f5-4485-a469-357f72a135b8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_095e966b-179f-4b75-a0d7-263b2dcb5d00" xlink:to="loc_srt_ConsolidatedEntitiesDomain_b8f79fa7-76f5-4485-a469-357f72a135b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SubsidiariesMember_66d25740-293a-48a3-9234-9b382ed7bd47" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SubsidiariesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_b8f79fa7-76f5-4485-a469-357f72a135b8" xlink:to="loc_srt_SubsidiariesMember_66d25740-293a-48a3-9234-9b382ed7bd47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonEquityTierOneCapitalRatio_f88c3a82-6e21-4572-88c4-166f2d3f4981" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapitalRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_CommonEquityTierOneCapitalRatio_f88c3a82-6e21-4572-88c4-166f2d3f4981" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum_c7a5c551-c515-4ef9-a450-4c924c4474d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum_c7a5c551-c515-4ef9-a450-4c924c4474d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankingRegulationCapitalConservationBufferCommonEquityTierOneRiskBasedCapitalActual_0c532a24-2a9a-4d49-830f-7f4017c207a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCapitalConservationBufferCommonEquityTierOneRiskBasedCapitalActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_BankingRegulationCapitalConservationBufferCommonEquityTierOneRiskBasedCapitalActual_0c532a24-2a9a-4d49-830f-7f4017c207a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets_bfd419a7-e8c1-4930-a7d5-25cc76857a80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets_bfd419a7-e8c1-4930-a7d5-25cc76857a80" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets_f0e546e9-7a04-46de-bf11-cec1e1e5ec50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets_f0e546e9-7a04-46de-bf11-cec1e1e5ec50" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankingRegulationCapitalConservationBufferTierOneRiskBasedCapitalActual_2e7967b0-1893-45be-ada2-39c16aa461f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCapitalConservationBufferTierOneRiskBasedCapitalActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_BankingRegulationCapitalConservationBufferTierOneRiskBasedCapitalActual_2e7967b0-1893-45be-ada2-39c16aa461f7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalToRiskWeightedAssets_ea593de0-fce7-4a3c-8c30-48a3688ebc95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_CapitalToRiskWeightedAssets_ea593de0-fce7-4a3c-8c30-48a3688ebc95" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets_67e600ef-9460-4e87-b93e-4169b765c3b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets_67e600ef-9460-4e87-b93e-4169b765c3b5" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankingRegulationCapitalConservationBufferTotalRiskBasedCapitalActual_9471d5d4-9c1d-4b76-a985-fa3e41333418" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCapitalConservationBufferTotalRiskBasedCapitalActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_BankingRegulationCapitalConservationBufferTotalRiskBasedCapitalActual_9471d5d4-9c1d-4b76-a985-fa3e41333418" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets_62bc32c2-3816-4d99-9296-a02465150820" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets_62bc32c2-3816-4d99-9296-a02465150820" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankingRegulationMaximumPayoutAmount_d8a42d77-4ac5-4bb3-81c5-da66642de0a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationMaximumPayoutAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_2620b293-62ed-4926-8ce5-fb7dc8dc2a0a" xlink:to="loc_us-gaap_BankingRegulationMaximumPayoutAmount_d8a42d77-4ac5-4bb3-81c5-da66642de0a0" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CapitalRequirementsandRestrictionsonRetainedEarningsScheduleofCapitalRatiosDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#CapitalRequirementsandRestrictionsonRetainedEarningsScheduleofCapitalRatiosDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CapitalRequirementsandRestrictionsonRetainedEarningsScheduleofCapitalRatiosDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_b803bfe0-4345-4993-92bc-c7a1d6eacf93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_b803bfe0-4345-4993-92bc-c7a1d6eacf93" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_15f152bf-591b-49ee-8b61-a3f8f3222938" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFederalHomeLoanBankAdvancesByBranchOfFHLBBankTable_b803bfe0-4345-4993-92bc-c7a1d6eacf93" xlink:to="loc_srt_ConsolidatedEntitiesAxis_15f152bf-591b-49ee-8b61-a3f8f3222938" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_15f152bf-591b-49ee-8b61-a3f8f3222938_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_15f152bf-591b-49ee-8b61-a3f8f3222938" xlink:to="loc_srt_ConsolidatedEntitiesDomain_15f152bf-591b-49ee-8b61-a3f8f3222938_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_25280337-8c9a-4031-b12b-044c940f8913" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_15f152bf-591b-49ee-8b61-a3f8f3222938" xlink:to="loc_srt_ConsolidatedEntitiesDomain_25280337-8c9a-4031-b12b-044c940f8913" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SubsidiariesMember_bc24b3dd-fc1a-416c-8918-5adb41ed3163" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SubsidiariesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_25280337-8c9a-4031-b12b-044c940f8913" xlink:to="loc_srt_SubsidiariesMember_bc24b3dd-fc1a-416c-8918-5adb41ed3163" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Capital_515ac937-44a0-4182-aee7-ced05264e0b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Capital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_Capital_515ac937-44a0-4182-aee7-ced05264e0b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalToRiskWeightedAssets_350b1c20-fb41-40f7-a7af-54ce9f5f4ce9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CapitalToRiskWeightedAssets_350b1c20-fb41-40f7-a7af-54ce9f5f4ce9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalRequiredForCapitalAdequacy_c2cdf5bf-e47b-46ea-adb0-3e03a97c5681" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredForCapitalAdequacy"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CapitalRequiredForCapitalAdequacy_c2cdf5bf-e47b-46ea-adb0-3e03a97c5681" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets_2fa843e6-4023-4b9e-b32a-bd9b991407a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets_2fa843e6-4023-4b9e-b32a-bd9b991407a3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapital_24fe082d-87f0-4197-a599-c1ab3d5e2e36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneRiskBasedCapital_24fe082d-87f0-4197-a599-c1ab3d5e2e36" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets_10b567d1-832f-48f2-895b-d9dc18fdb06b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets_10b567d1-832f-48f2-895b-d9dc18fdb06b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy_16044763-d318-4fad-89ae-133090029b4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy_16044763-d318-4fad-89ae-133090029b4d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets_d03dc9ea-893d-46ee-a0ad-f72a05a877ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets_d03dc9ea-893d-46ee-a0ad-f72a05a877ef" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonEquityTierOneCapital_a5e206aa-1fba-488c-a048-1079f0955d2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CommonEquityTierOneCapital_a5e206aa-1fba-488c-a048-1079f0955d2f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonEquityTierOneCapitalRatio_56887dfa-7edf-4e02-932e-4b8b94a4f8f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapitalRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CommonEquityTierOneCapitalRatio_56887dfa-7edf-4e02-932e-4b8b94a4f8f1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonEquityTierOneCapitalRequiredForCapitalAdequacy_e093cb9d-c28d-493b-a905-3a21cad0a58b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapitalRequiredForCapitalAdequacy"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CommonEquityTierOneCapitalRequiredForCapitalAdequacy_e093cb9d-c28d-493b-a905-3a21cad0a58b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum_81723d62-bb79-49e7-927e-6a765c1e6e0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum_81723d62-bb79-49e7-927e-6a765c1e6e0a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapital_027f267f-6958-44c4-b351-89162c64ab72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneLeverageCapital_027f267f-6958-44c4-b351-89162c64ab72" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapitalToAverageAssets_58f290fc-4763-43f9-8b19-93b068e3f058" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalToAverageAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneLeverageCapitalToAverageAssets_58f290fc-4763-43f9-8b19-93b068e3f058" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy_d742fbae-31e2-48a3-af03-33e5b1f36b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy_d742fbae-31e2-48a3-af03-33e5b1f36b4f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets_45bf1195-0f6e-4531-8591-fde1e6d90524" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets_45bf1195-0f6e-4531-8591-fde1e6d90524" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalRequiredToBeWellCapitalized_5b40925b-3e65-412c-a66d-8b6aaff0f121" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredToBeWellCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CapitalRequiredToBeWellCapitalized_5b40925b-3e65-412c-a66d-8b6aaff0f121" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets_8f8aa4b7-7d23-4b25-84a1-56b4cc810d0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets_8f8aa4b7-7d23-4b25-84a1-56b4cc810d0c" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized_013de352-2521-45d1-94a3-755a8644d38f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized_013de352-2521-45d1-94a3-755a8644d38f" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets_968a0742-6165-4074-8202-ed7f8f2537ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets_968a0742-6165-4074-8202-ed7f8f2537ac" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommonEquityTierRequiredToBeCategorizedWellCapitalizedAmount_85213bc7-f8f7-4b03-8664-2052f8ae534f" xlink:href="fsbc-20251231.xsd#fsbc_CommonEquityTierRequiredToBeCategorizedWellCapitalizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_fsbc_CommonEquityTierRequiredToBeCategorizedWellCapitalizedAmount_85213bc7-f8f7-4b03-8664-2052f8ae534f" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommonEquityTierRequiredToBeCategorizedWellCapitalizedRatioAsPercent_1130c4d3-0442-4138-8c28-d5a2f042a4db" xlink:href="fsbc-20251231.xsd#fsbc_CommonEquityTierRequiredToBeCategorizedWellCapitalizedRatioAsPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_fsbc_CommonEquityTierRequiredToBeCategorizedWellCapitalizedRatioAsPercent_1130c4d3-0442-4138-8c28-d5a2f042a4db" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized_502cffe9-f729-4221-88b4-558b57e3f9c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized_502cffe9-f729-4221-88b4-558b57e3f9c8" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets_ef284462-bc4c-432e-a8c4-eaf8d60d3ef8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems_87bcbe79-d9dd-49b6-9d75-3cad8ae154ce" xlink:to="loc_us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets_ef284462-bc4c-432e-a8c4-eaf8d60d3ef8" xlink:type="arc" order="23"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesScheduleofFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#FairValueofAssetsandLiabilitiesScheduleofFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesScheduleofFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_fe57fd79-fee8-4670-b2d7-3850398d8593" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_35143495-2a0c-447d-a9d0-0c2232961514" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_fe57fd79-fee8-4670-b2d7-3850398d8593" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_35143495-2a0c-447d-a9d0-0c2232961514" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_88fa9b77-5640-49d4-a138-db99a10d20ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_35143495-2a0c-447d-a9d0-0c2232961514" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_88fa9b77-5640-49d4-a138-db99a10d20ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_88fa9b77-5640-49d4-a138-db99a10d20ee_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_88fa9b77-5640-49d4-a138-db99a10d20ee" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_88fa9b77-5640-49d4-a138-db99a10d20ee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_d82bb592-3e96-40fa-931f-52efd80ef507" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_88fa9b77-5640-49d4-a138-db99a10d20ee" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_d82bb592-3e96-40fa-931f-52efd80ef507" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_718eb364-b9e5-491d-8410-7391437d936c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_d82bb592-3e96-40fa-931f-52efd80ef507" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_718eb364-b9e5-491d-8410-7391437d936c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3f69941-d7fb-4fe5-aa34-3bd6328e4851" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_35143495-2a0c-447d-a9d0-0c2232961514" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3f69941-d7fb-4fe5-aa34-3bd6328e4851" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f3f69941-d7fb-4fe5-aa34-3bd6328e4851_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3f69941-d7fb-4fe5-aa34-3bd6328e4851" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f3f69941-d7fb-4fe5-aa34-3bd6328e4851_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7be6b83a-3a99-4243-8e2a-11ef5d6e27c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3f69941-d7fb-4fe5-aa34-3bd6328e4851" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7be6b83a-3a99-4243-8e2a-11ef5d6e27c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_32cc0411-9da0-4207-8e42-6730fae700a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7be6b83a-3a99-4243-8e2a-11ef5d6e27c7" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_32cc0411-9da0-4207-8e42-6730fae700a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_cb887417-ae41-4d59-9937-1447f749d9e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7be6b83a-3a99-4243-8e2a-11ef5d6e27c7" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_cb887417-ae41-4d59-9937-1447f749d9e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_456dc559-46fb-4975-82f1-3e4d494cc8d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7be6b83a-3a99-4243-8e2a-11ef5d6e27c7" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_456dc559-46fb-4975-82f1-3e4d494cc8d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_b3f68f89-4dfe-4b84-a577-4ccaefc07bd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_fe57fd79-fee8-4670-b2d7-3850398d8593" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_b3f68f89-4dfe-4b84-a577-4ccaefc07bd4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#FairValueofAssetsandLiabilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_ab304012-0582-4e41-8496-d894d92f14d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_68c2682b-8b37-46be-a8e1-dd7400b39e75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_ab304012-0582-4e41-8496-d894d92f14d1" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_68c2682b-8b37-46be-a8e1-dd7400b39e75" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_37474fdd-7296-4e37-a897-9f94a7f2dfc7" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_68c2682b-8b37-46be-a8e1-dd7400b39e75" xlink:to="loc_srt_RangeAxis_37474fdd-7296-4e37-a897-9f94a7f2dfc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_37474fdd-7296-4e37-a897-9f94a7f2dfc7_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_37474fdd-7296-4e37-a897-9f94a7f2dfc7" xlink:to="loc_srt_RangeMember_37474fdd-7296-4e37-a897-9f94a7f2dfc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a28963a8-2d98-4f66-a899-dd3e27019652" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_37474fdd-7296-4e37-a897-9f94a7f2dfc7" xlink:to="loc_srt_RangeMember_a28963a8-2d98-4f66-a899-dd3e27019652" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_f96ea788-6664-4e72-ae75-b4dacea0b8ce" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a28963a8-2d98-4f66-a899-dd3e27019652" xlink:to="loc_srt_MinimumMember_f96ea788-6664-4e72-ae75-b4dacea0b8ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_19f6f630-ce91-4966-b1f6-aa21e1808baf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a28963a8-2d98-4f66-a899-dd3e27019652" xlink:to="loc_srt_MaximumMember_19f6f630-ce91-4966-b1f6-aa21e1808baf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SellingAndCommissionCostsPercentOfProfit_d6eb8590-e9aa-4b97-ae18-8f5f6352b3c7" xlink:href="fsbc-20251231.xsd#fsbc_SellingAndCommissionCostsPercentOfProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_ab304012-0582-4e41-8496-d894d92f14d1" xlink:to="loc_fsbc_SellingAndCommissionCostsPercentOfProfit_d6eb8590-e9aa-4b97-ae18-8f5f6352b3c7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesScheduleofFairValueEstimatesforFinancialInstrumentsbyBalanceSheetGroupingDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#FairValueofAssetsandLiabilitiesScheduleofFairValueEstimatesforFinancialInstrumentsbyBalanceSheetGroupingDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesScheduleofFairValueEstimatesforFinancialInstrumentsbyBalanceSheetGroupingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_c531e3ac-61f2-48a3-93b4-3c5941a4fbea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a7657b91-3184-4b0f-b956-36c4c68286fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_c531e3ac-61f2-48a3-93b4-3c5941a4fbea" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a7657b91-3184-4b0f-b956-36c4c68286fc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_7ac0beb8-4875-4afd-80ae-e51d82c203a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a7657b91-3184-4b0f-b956-36c4c68286fc" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_7ac0beb8-4875-4afd-80ae-e51d82c203a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_7ac0beb8-4875-4afd-80ae-e51d82c203a9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_7ac0beb8-4875-4afd-80ae-e51d82c203a9" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_7ac0beb8-4875-4afd-80ae-e51d82c203a9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_7f52d948-f558-4185-b65b-37feb9ecc15e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_7ac0beb8-4875-4afd-80ae-e51d82c203a9" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_7f52d948-f558-4185-b65b-37feb9ecc15e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_86903460-f91f-41f0-b981-485df4158f76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_7f52d948-f558-4185-b65b-37feb9ecc15e" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_86903460-f91f-41f0-b981-485df4158f76" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_c573848f-453b-4d38-af8e-4f32fbf257e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_86903460-f91f-41f0-b981-485df4158f76" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_c573848f-453b-4d38-af8e-4f32fbf257e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_6517a592-2d21-47b4-b73a-7bf85e6aa3bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_86903460-f91f-41f0-b981-485df4158f76" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_6517a592-2d21-47b4-b73a-7bf85e6aa3bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_131fdd0b-7ef8-43dd-a408-df3b953eba29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a7657b91-3184-4b0f-b956-36c4c68286fc" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_131fdd0b-7ef8-43dd-a408-df3b953eba29" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_131fdd0b-7ef8-43dd-a408-df3b953eba29_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_131fdd0b-7ef8-43dd-a408-df3b953eba29" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_131fdd0b-7ef8-43dd-a408-df3b953eba29_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_96043132-a388-432b-8c62-18e2cceb5353" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_131fdd0b-7ef8-43dd-a408-df3b953eba29" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_96043132-a388-432b-8c62-18e2cceb5353" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_390b3d9a-ca59-4169-a7d2-b02b06f48e5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_96043132-a388-432b-8c62-18e2cceb5353" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_390b3d9a-ca59-4169-a7d2-b02b06f48e5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_3f4bec62-26ae-40f1-a080-533a7facb7d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_96043132-a388-432b-8c62-18e2cceb5353" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_3f4bec62-26ae-40f1-a080-533a7facb7d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_6713558e-852f-4953-a6f5-d9c3b4df7e07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_96043132-a388-432b-8c62-18e2cceb5353" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_6713558e-852f-4953-a6f5-d9c3b4df7e07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:href="fsbc-20251231.xsd#fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_c531e3ac-61f2-48a3-93b4-3c5941a4fbea" xlink:to="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_837c0b8e-68db-42b8-9dd1-1f231cae88b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_837c0b8e-68db-42b8-9dd1-1f231cae88b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TimeDepositAtFairValueDisclosure_317e8803-8438-406f-ae28-0dc41a465cce" xlink:href="fsbc-20251231.xsd#fsbc_TimeDepositAtFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:to="loc_fsbc_TimeDepositAtFairValueDisclosure_317e8803-8438-406f-ae28-0dc41a465cce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_3ae0712b-bf94-41f8-b8b7-754897da8892" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_3ae0712b-bf94-41f8-b8b7-754897da8892" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_e660e653-35b9-4353-9cbb-e025aebd187d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_e660e653-35b9-4353-9cbb-e025aebd187d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansHeldForSaleFairValueDisclosure_97d7feb5-a573-4e1d-bd06-376f70f98a12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansHeldForSaleFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:to="loc_us-gaap_LoansHeldForSaleFairValueDisclosure_97d7feb5-a573-4e1d-bd06-376f70f98a12" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableFairValueDisclosure_be59b860-e47f-4d52-ad51-e915dcdf1468" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_a957e774-208e-4ab9-92b7-9e690b2176f1" xlink:to="loc_us-gaap_LoansReceivableFairValueDisclosure_be59b860-e47f-4d52-ad51-e915dcdf1468" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_5dd0f5dc-0514-479d-a67c-ef42678eaa07" xlink:href="fsbc-20251231.xsd#fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_c531e3ac-61f2-48a3-93b4-3c5941a4fbea" xlink:to="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_5dd0f5dc-0514-479d-a67c-ef42678eaa07" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimeDeposits_3ffd53db-a5f6-4ec2-a06b-91c2c0a6a174" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_5dd0f5dc-0514-479d-a67c-ef42678eaa07" xlink:to="loc_us-gaap_TimeDeposits_3ffd53db-a5f6-4ec2-a06b-91c2c0a6a174" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations_3a607c03-7fd3-4dd8-a82a-50ff2de0897f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_5dd0f5dc-0514-479d-a67c-ef42678eaa07" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations_3a607c03-7fd3-4dd8-a82a-50ff2de0897f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/ParentCompanyOnlyCondensedFinancialInformationBalanceSheetDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#ParentCompanyOnlyCondensedFinancialInformationBalanceSheetDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/ParentCompanyOnlyCondensedFinancialInformationBalanceSheetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_aa503387-9695-487f-8f81-7d244072dbdc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d6e7c091-ee7a-4e1c-9c30-d6302a1e38d5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_aa503387-9695-487f-8f81-7d244072dbdc" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d6e7c091-ee7a-4e1c-9c30-d6302a1e38d5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_7c83715f-fe61-4594-b8b5-6dac48ad2ecf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d6e7c091-ee7a-4e1c-9c30-d6302a1e38d5" xlink:to="loc_srt_ConsolidatedEntitiesAxis_7c83715f-fe61-4594-b8b5-6dac48ad2ecf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_7c83715f-fe61-4594-b8b5-6dac48ad2ecf_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_7c83715f-fe61-4594-b8b5-6dac48ad2ecf" xlink:to="loc_srt_ConsolidatedEntitiesDomain_7c83715f-fe61-4594-b8b5-6dac48ad2ecf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_3f28c6c0-d80c-4a8b-9522-c6afa8c2bff8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_7c83715f-fe61-4594-b8b5-6dac48ad2ecf" xlink:to="loc_srt_ConsolidatedEntitiesDomain_3f28c6c0-d80c-4a8b-9522-c6afa8c2bff8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_4d1ce905-5f31-4712-be2b-1d4ade31861b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_3f28c6c0-d80c-4a8b-9522-c6afa8c2bff8" xlink:to="loc_srt_ParentCompanyMember_4d1ce905-5f31-4712-be2b-1d4ade31861b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_0be352fa-5fb0-41de-a0c2-f83b5b9c7ff7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_aa503387-9695-487f-8f81-7d244072dbdc" xlink:to="loc_us-gaap_AssetsAbstract_0be352fa-5fb0-41de-a0c2-f83b5b9c7ff7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_db1cfbd1-a251-41d5-9dd8-66407752aeac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0be352fa-5fb0-41de-a0c2-f83b5b9c7ff7" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_db1cfbd1-a251-41d5-9dd8-66407752aeac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_InvestmentInBankingSubsidiary_2b2c778f-e794-4621-aea5-ec602ab0cfed" xlink:href="fsbc-20251231.xsd#fsbc_InvestmentInBankingSubsidiary"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0be352fa-5fb0-41de-a0c2-f83b5b9c7ff7" xlink:to="loc_fsbc_InvestmentInBankingSubsidiary_2b2c778f-e794-4621-aea5-ec602ab0cfed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssets_2734a037-c0af-4d9f-bc51-85775f418549" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0be352fa-5fb0-41de-a0c2-f83b5b9c7ff7" xlink:to="loc_us-gaap_OtherAssets_2734a037-c0af-4d9f-bc51-85775f418549" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_a67d2ffd-3895-42be-b052-e30a90565ef2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0be352fa-5fb0-41de-a0c2-f83b5b9c7ff7" xlink:to="loc_us-gaap_Assets_a67d2ffd-3895-42be-b052-e30a90565ef2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_1bfed171-1e1b-486a-9274-750d28d702a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_aa503387-9695-487f-8f81-7d244072dbdc" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_1bfed171-1e1b-486a-9274-750d28d702a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebt_3b923e8f-4731-4fdd-a9e1-bfc9d9c23ff1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_1bfed171-1e1b-486a-9274-750d28d702a0" xlink:to="loc_us-gaap_SubordinatedDebt_3b923e8f-4731-4fdd-a9e1-bfc9d9c23ff1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_207a159c-879e-4d41-92d1-c1b02cb9b667" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_1bfed171-1e1b-486a-9274-750d28d702a0" xlink:to="loc_us-gaap_StockholdersEquity_207a159c-879e-4d41-92d1-c1b02cb9b667" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_fae08828-aa79-471d-a541-7cc67b19fbae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_1bfed171-1e1b-486a-9274-750d28d702a0" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_fae08828-aa79-471d-a541-7cc67b19fbae" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/ParentCompanyOnlyCondensedFinancialInformationStatementofIncomeDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#ParentCompanyOnlyCondensedFinancialInformationStatementofIncomeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/ParentCompanyOnlyCondensedFinancialInformationStatementofIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_1784cddb-a4e9-4b0f-8618-48d76b6a83c3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_1784cddb-a4e9-4b0f-8618-48d76b6a83c3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_1413a64e-e2dc-49f4-a77f-6e5a1c4ec6b8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_1784cddb-a4e9-4b0f-8618-48d76b6a83c3" xlink:to="loc_srt_ConsolidatedEntitiesAxis_1413a64e-e2dc-49f4-a77f-6e5a1c4ec6b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_1413a64e-e2dc-49f4-a77f-6e5a1c4ec6b8_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_1413a64e-e2dc-49f4-a77f-6e5a1c4ec6b8" xlink:to="loc_srt_ConsolidatedEntitiesDomain_1413a64e-e2dc-49f4-a77f-6e5a1c4ec6b8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_0e99587e-be41-4370-8cef-16a6fe30ef57" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_1413a64e-e2dc-49f4-a77f-6e5a1c4ec6b8" xlink:to="loc_srt_ConsolidatedEntitiesDomain_0e99587e-be41-4370-8cef-16a6fe30ef57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_ab731f6d-facd-46a7-8176-439b451f59ff" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_0e99587e-be41-4370-8cef-16a6fe30ef57" xlink:to="loc_srt_ParentCompanyMember_ab731f6d-facd-46a7-8176-439b451f59ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndDividendIncomeOperating_6f48dca7-4602-401a-89d2-bd17c0e2161c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestAndDividendIncomeOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_InterestAndDividendIncomeOperating_6f48dca7-4602-401a-89d2-bd17c0e2161c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIncome_019ba0e3-8e6c-79d0-9bc7-5c8ed981c509" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_OtherIncome_019ba0e3-8e6c-79d0-9bc7-5c8ed981c509" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_10d161a7-c60f-4294-8fdf-4545f62eeae9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_InterestExpense_10d161a7-c60f-4294-8fdf-4545f62eeae9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherExpenses_77a0340b-9f60-4295-9339-7b50ca9fbb67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_OtherExpenses_77a0340b-9f60-4295-9339-7b50ca9fbb67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_1d089636-2872-47ec-995c-1637de13c72d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_1d089636-2872-47ec-995c-1637de13c72d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_5a15b334-f4e3-4c25-a8ce-e3c90d2f35a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_5a15b334-f4e3-4c25-a8ce-e3c90d2f35a8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SubsidiaryOrEquityMethodInvesteeUndistributedBankingSubsidiaryIncome_2826be03-753a-4c31-bc73-ccea114b62d1" xlink:href="fsbc-20251231.xsd#fsbc_SubsidiaryOrEquityMethodInvesteeUndistributedBankingSubsidiaryIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_fsbc_SubsidiaryOrEquityMethodInvesteeUndistributedBankingSubsidiaryIncome_2826be03-753a-4c31-bc73-ccea114b62d1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_4092f4b1-8aeb-4aaf-b7d8-fabdc47afc02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_74d24398-1b1d-4fc5-8f96-1190df4ac3cd" xlink:to="loc_us-gaap_NetIncomeLoss_4092f4b1-8aeb-4aaf-b7d8-fabdc47afc02" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/ParentCompanyOnlyCondensedFinancialInformationStatementofCashFlowsDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#ParentCompanyOnlyCondensedFinancialInformationStatementofCashFlowsDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/ParentCompanyOnlyCondensedFinancialInformationStatementofCashFlowsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d8e0881b-3c1e-42d7-9383-dbcde23de8b2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d8e0881b-3c1e-42d7-9383-dbcde23de8b2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_6d8647b1-e135-47ff-85e9-22da8b63c944" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d8e0881b-3c1e-42d7-9383-dbcde23de8b2" xlink:to="loc_srt_ConsolidatedEntitiesAxis_6d8647b1-e135-47ff-85e9-22da8b63c944" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_6d8647b1-e135-47ff-85e9-22da8b63c944_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_6d8647b1-e135-47ff-85e9-22da8b63c944" xlink:to="loc_srt_ConsolidatedEntitiesDomain_6d8647b1-e135-47ff-85e9-22da8b63c944_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_0b64fdad-69b0-4371-a922-c5ab6c529208" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_6d8647b1-e135-47ff-85e9-22da8b63c944" xlink:to="loc_srt_ConsolidatedEntitiesDomain_0b64fdad-69b0-4371-a922-c5ab6c529208" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_d3f5cb05-aabd-4bd1-883e-a411df65c0ef" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_0b64fdad-69b0-4371-a922-c5ab6c529208" xlink:to="loc_srt_ParentCompanyMember_d3f5cb05-aabd-4bd1-883e-a411df65c0ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_9e90e28a-c5b5-4f42-a8a3-e5454f097efb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_9e90e28a-c5b5-4f42-a8a3-e5454f097efb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_dccc4a22-2b23-4d41-bf64-3df21cf5dc49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_9e90e28a-c5b5-4f42-a8a3-e5454f097efb" xlink:to="loc_us-gaap_NetIncomeLoss_dccc4a22-2b23-4d41-bf64-3df21cf5dc49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_9e90e28a-c5b5-4f42-a8a3-e5454f097efb" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_cb0cb566-c9a9-4fb3-8621-5009c6be3115" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:to="loc_us-gaap_ShareBasedCompensation_cb0cb566-c9a9-4fb3-8621-5009c6be3115" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions_ba2f8718-55ef-43df-8aa1-94b51962462f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions_ba2f8718-55ef-43df-8aa1-94b51962462f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_e39c3a03-1f4a-4c6f-bf39-8e9d0e87c16f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_e39c3a03-1f4a-4c6f-bf39-8e9d0e87c16f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_f996b223-8c86-41b8-ab1b-055d25eeb568" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_f996b223-8c86-41b8-ab1b-055d25eeb568" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_8b5f75af-3850-4bfa-a08b-3d5f9108086a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_8d385936-8aec-499e-bb6e-2cb37e288f4c" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_8b5f75af-3850-4bfa-a08b-3d5f9108086a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_c1741760-ec51-4202-9f0d-4631b0a87b1f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_9e90e28a-c5b5-4f42-a8a3-e5454f097efb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_c1741760-ec51-4202-9f0d-4631b0a87b1f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_0e327c40-713d-4f0f-b59f-2db1d524f59b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_0e327c40-713d-4f0f-b59f-2db1d524f59b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates_c7cd0c75-d73f-4b38-8376-9180b9ac4245" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_0e327c40-713d-4f0f-b59f-2db1d524f59b" xlink:to="loc_us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates_c7cd0c75-d73f-4b38-8376-9180b9ac4245" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_55fcd108-ca85-4ca8-a392-55fa3490502c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_0e327c40-713d-4f0f-b59f-2db1d524f59b" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_55fcd108-ca85-4ca8-a392-55fa3490502c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_0328686e-d07c-4a5f-943a-d3f5c5136cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_0328686e-d07c-4a5f-943a-d3f5c5136cd8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_ff25b455-e554-4333-8c44-e262a936e38a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_0328686e-d07c-4a5f-943a-d3f5c5136cd8" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_ff25b455-e554-4333-8c44-e262a936e38a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividends_76070d2a-b039-487f-b250-3885e4a6c331" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_0328686e-d07c-4a5f-943a-d3f5c5136cd8" xlink:to="loc_us-gaap_PaymentsOfDividends_76070d2a-b039-487f-b250-3885e4a6c331" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_ca947781-c3d5-43fc-884d-ed71a3e88720" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_0328686e-d07c-4a5f-943a-d3f5c5136cd8" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_ca947781-c3d5-43fc-884d-ed71a3e88720" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_798e4aed-44e7-4ba1-9fec-2b775d8ea0a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_798e4aed-44e7-4ba1-9fec-2b775d8ea0a3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_b076658a-e6ec-418f-97a3-06de7eef3929" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_146126bf-8cdb-45ee-b2d2-bf68f17f5bde" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_b076658a-e6ec-418f-97a3-06de7eef3929" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_c80891b8-8460-447a-a0d5-98009d0fd3fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="fsbc-20251231.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_a59d4e61-c71a-4a56-b4f1-e12df8226703" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_af1aea6e-4a05-42fe-b4bf-0195317237de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_a59d4e61-c71a-4a56-b4f1-e12df8226703" xlink:to="loc_us-gaap_SubsequentEventTable_af1aea6e-4a05-42fe-b4bf-0195317237de" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_49aeeb77-8d71-4b5c-bca5-b6b0e892a646" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_af1aea6e-4a05-42fe-b4bf-0195317237de" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_49aeeb77-8d71-4b5c-bca5-b6b0e892a646" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_49aeeb77-8d71-4b5c-bca5-b6b0e892a646_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_49aeeb77-8d71-4b5c-bca5-b6b0e892a646" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_49aeeb77-8d71-4b5c-bca5-b6b0e892a646_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_0584ac0d-d9b9-4064-a862-f374712dcf77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_49aeeb77-8d71-4b5c-bca5-b6b0e892a646" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_0584ac0d-d9b9-4064-a862-f374712dcf77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_68f919f1-ec90-4d72-ad25-8e4aabfd9401" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_0584ac0d-d9b9-4064-a862-f374712dcf77" xlink:to="loc_us-gaap_SubsequentEventMember_68f919f1-ec90-4d72-ad25-8e4aabfd9401" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_d43bf736-6fc9-4e02-89dc-53e2f71307de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_a59d4e61-c71a-4a56-b4f1-e12df8226703" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_d43bf736-6fc9-4e02-89dc-53e2f71307de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividendsCommonStock_a83eab69-084c-4f82-9d9e-29fd1465cd52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_a59d4e61-c71a-4a56-b4f1-e12df8226703" xlink:to="loc_us-gaap_PaymentsOfDividendsCommonStock_a83eab69-084c-4f82-9d9e-29fd1465cd52" xlink:type="arc" order="1"/>
  </link:definitionLink>
</link:linkbase>
