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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e172a-6db9-7e81-813c-fe3a0e9f5494,g:5ad9cfd9-4e1b-456c-bdcb-2ad3645fd1e3-->
<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://www.fastenal.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="fast-20260630.xsd#CondensedConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e172a-6e74-7936-82ff-ef619b579724" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e172a-6e74-71a1-a9d2-75cf003358cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e172a-6e74-7936-82ff-ef619b579724" xlink:to="loc_us-gaap_StatementTable_019e172a-6e74-71a1-a9d2-75cf003358cc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e172a-6e74-7b2b-a3c4-1cf3e6ba2214" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e172a-6e74-71a1-a9d2-75cf003358cc" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e172a-6e74-7b2b-a3c4-1cf3e6ba2214" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7b2b-a3c4-1cf3e6ba2214_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e172a-6e74-7b2b-a3c4-1cf3e6ba2214" xlink:to="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7b2b-a3c4-1cf3e6ba2214_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7bd4-9292-b351698b67dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e172a-6e74-7b2b-a3c4-1cf3e6ba2214" xlink:to="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7bd4-9292-b351698b67dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e172a-6e74-7c9d-bcb0-c4ba22c39f95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7bd4-9292-b351698b67dc" xlink:to="loc_us-gaap_CommonStockMember_019e172a-6e74-7c9d-bcb0-c4ba22c39f95" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e172a-6e74-7b20-a9e7-024e613ec1ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7bd4-9292-b351698b67dc" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e172a-6e74-7b20-a9e7-024e613ec1ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e172a-6e74-74aa-b746-e566f3a606bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7bd4-9292-b351698b67dc" xlink:to="loc_us-gaap_RetainedEarningsMember_019e172a-6e74-74aa-b746-e566f3a606bc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e172a-6e74-796c-bf99-e6ce519d21d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e172a-6e74-7bd4-9292-b351698b67dc" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e172a-6e74-796c-bf99-e6ce519d21d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e172a-6e74-7936-82ff-ef619b579724" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e172a-6e74-72f8-8c36-9109be8eab04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_StockholdersEquity_019e172a-6e74-72f8-8c36-9109be8eab04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_019e172a-6e74-7786-94a1-c332f3244690" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_019e172a-6e74-7786-94a1-c332f3244690" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e172a-6e74-7ac5-a810-6ac8036c827d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e172a-6e74-7ac5-a810-6ac8036c827d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e172a-6e74-7dd8-9a3b-a2b8b52de324" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e172a-6e74-7dd8-9a3b-a2b8b52de324" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e172a-6e74-776c-90cb-7e331f752c72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_NetIncomeLoss_019e172a-6e74-776c-90cb-7e331f752c72" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCash_019e172a-6e74-724f-81ca-bee25903d4a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_DividendsCash_019e172a-6e74-724f-81ca-bee25903d4a3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019e172a-6e74-7e28-a084-ea700e70480f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e172a-6e74-7a06-89e4-64e2e20679d1" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019e172a-6e74-7e28-a084-ea700e70480f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e172a-6e74-7592-bb03-5a3999af5585" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/RevenueRevenuesbyGeographicAreasDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#RevenueRevenuesbyGeographicAreasDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/RevenueRevenuesbyGeographicAreasDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-79d9-a9ff-f050a7209821" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7868-b9d9-977edcf54fdf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-79d9-a9ff-f050a7209821" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7868-b9d9-977edcf54fdf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e172a-6e74-7a1e-bc95-0a35a0c79ae8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7868-b9d9-977edcf54fdf" xlink:to="loc_srt_StatementGeographicalAxis_019e172a-6e74-7a1e-bc95-0a35a0c79ae8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e172a-6e74-7a1e-bc95-0a35a0c79ae8_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e172a-6e74-7a1e-bc95-0a35a0c79ae8" xlink:to="loc_srt_SegmentGeographicalDomain_019e172a-6e74-7a1e-bc95-0a35a0c79ae8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e172a-6e74-739a-b76d-9ab7ead602a3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e172a-6e74-7a1e-bc95-0a35a0c79ae8" xlink:to="loc_srt_SegmentGeographicalDomain_019e172a-6e74-739a-b76d-9ab7ead602a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e172a-6e74-7ce5-b220-5f7b4f0e1523" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e172a-6e74-739a-b76d-9ab7ead602a3" xlink:to="loc_country_US_019e172a-6e74-7ce5-b220-5f7b4f0e1523" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_CanadaandMexicoMember_019e172a-6e74-70f8-997b-1fc8189579ba" xlink:href="fast-20260630.xsd#fast_CanadaandMexicoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e172a-6e74-739a-b76d-9ab7ead602a3" xlink:to="loc_fast_CanadaandMexicoMember_019e172a-6e74-70f8-997b-1fc8189579ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_NonNorthAmericaMember_019e172a-6e74-797c-ae4a-8270ec5c9743" xlink:href="fast-20260630.xsd#fast_NonNorthAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e172a-6e74-739a-b76d-9ab7ead602a3" xlink:to="loc_fast_NonNorthAmericaMember_019e172a-6e74-797c-ae4a-8270ec5c9743" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7bbc-b300-d9c5753dfc1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7868-b9d9-977edcf54fdf" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7bbc-b300-d9c5753dfc1e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7bbc-b300-d9c5753dfc1e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7bbc-b300-d9c5753dfc1e" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7bbc-b300-d9c5753dfc1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7001-b8f0-bc7a85eb1530" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7bbc-b300-d9c5753dfc1e" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7001-b8f0-bc7a85eb1530" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_019e172a-6e74-72eb-81ee-019815fd0dc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7001-b8f0-bc7a85eb1530" xlink:to="loc_us-gaap_SalesRevenueNetMember_019e172a-6e74-72eb-81ee-019815fd0dc5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-7d79-bba8-e766fe8ade45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7868-b9d9-977edcf54fdf" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-7d79-bba8-e766fe8ade45" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7d79-bba8-e766fe8ade45_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-7d79-bba8-e766fe8ade45" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7d79-bba8-e766fe8ade45_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7777-977e-0a778770ddf2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-7d79-bba8-e766fe8ade45" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7777-977e-0a778770ddf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeographicConcentrationRiskMember_019e172a-6e74-77cd-b7a0-aa4639631f6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeographicConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7777-977e-0a778770ddf2" xlink:to="loc_us-gaap_GeographicConcentrationRiskMember_019e172a-6e74-77cd-b7a0-aa4639631f6d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e172a-6e74-7ef9-b7c8-5e10fde6e234" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-79d9-a9ff-f050a7209821" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e172a-6e74-7ef9-b7c8-5e10fde6e234" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019e172a-6e74-7754-b109-6ed8f3836fd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-79d9-a9ff-f050a7209821" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019e172a-6e74-7754-b109-6ed8f3836fd0" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/RevenuePercentagesofSalesbyEndMarketDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#RevenuePercentagesofSalesbyEndMarketDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/RevenuePercentagesofSalesbyEndMarketDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-7896-b0ba-cb05bdf046da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7e38-b8ac-2e46c96d79f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-7896-b0ba-cb05bdf046da" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7e38-b8ac-2e46c96d79f4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019e172a-6e74-7a0b-a862-f89b4b49bc04" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7e38-b8ac-2e46c96d79f4" xlink:to="loc_srt_MajorCustomersAxis_019e172a-6e74-7a0b-a862-f89b4b49bc04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-7a0b-a862-f89b4b49bc04_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019e172a-6e74-7a0b-a862-f89b4b49bc04" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-7a0b-a862-f89b4b49bc04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-74c9-9123-87e8aa69acd7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019e172a-6e74-7a0b-a862-f89b4b49bc04" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-74c9-9123-87e8aa69acd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_ManufacturingCustomersMember_019e172a-6e74-75b7-9f8d-8c47819948a6" xlink:href="fast-20260630.xsd#fast_ManufacturingCustomersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-74c9-9123-87e8aa69acd7" xlink:to="loc_fast_ManufacturingCustomersMember_019e172a-6e74-75b7-9f8d-8c47819948a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_NonResidentialConstructionCustomersMember_019e172a-6e74-7d0c-8f86-f8a790a73d4d" xlink:href="fast-20260630.xsd#fast_NonResidentialConstructionCustomersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-74c9-9123-87e8aa69acd7" xlink:to="loc_fast_NonResidentialConstructionCustomersMember_019e172a-6e74-7d0c-8f86-f8a790a73d4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_OtherCustomersMember_019e172a-6e74-7734-a9a7-90ca187184a3" xlink:href="fast-20260630.xsd#fast_OtherCustomersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e172a-6e74-74c9-9123-87e8aa69acd7" xlink:to="loc_fast_OtherCustomersMember_019e172a-6e74-7734-a9a7-90ca187184a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-70ae-bae6-93455d41504e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7e38-b8ac-2e46c96d79f4" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-70ae-bae6-93455d41504e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-70ae-bae6-93455d41504e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-70ae-bae6-93455d41504e" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-70ae-bae6-93455d41504e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-77f8-83b4-a6f32f4ab81c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-70ae-bae6-93455d41504e" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-77f8-83b4-a6f32f4ab81c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019e172a-6e74-7215-a1d3-c64cb8c74fc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-77f8-83b4-a6f32f4ab81c" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019e172a-6e74-7215-a1d3-c64cb8c74fc5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7071-a298-95f56f2acdc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e172a-6e74-7e38-b8ac-2e46c96d79f4" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7071-a298-95f56f2acdc7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7071-a298-95f56f2acdc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7071-a298-95f56f2acdc7" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7071-a298-95f56f2acdc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-74d5-aeeb-54ee5ea84fd6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7071-a298-95f56f2acdc7" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-74d5-aeeb-54ee5ea84fd6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_019e172a-6e74-7c6e-86eb-22a362403b4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-74d5-aeeb-54ee5ea84fd6" xlink:to="loc_us-gaap_SalesRevenueNetMember_019e172a-6e74-7c6e-86eb-22a362403b4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019e172a-6e74-7472-a8a2-379f6c570d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e172a-6e74-7896-b0ba-cb05bdf046da" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019e172a-6e74-7472-a8a2-379f6c570d4c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/RevenuePercentagesofSalesbyProductLineDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#RevenuePercentagesofSalesbyProductLineDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/RevenuePercentagesofSalesbyProductLineDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019e172a-6e74-7db2-8a34-6ac20a7da4ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019e172a-6e74-7b71-944a-500d84fb1efb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e172a-6e74-7db2-8a34-6ac20a7da4ae" xlink:to="loc_us-gaap_ConcentrationRiskTable_019e172a-6e74-7b71-944a-500d84fb1efb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e172a-6e74-7d29-8f83-515bbc062e25" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019e172a-6e74-7b71-944a-500d84fb1efb" xlink:to="loc_srt_ProductOrServiceAxis_019e172a-6e74-7d29-8f83-515bbc062e25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7d29-8f83-515bbc062e25_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e172a-6e74-7d29-8f83-515bbc062e25" xlink:to="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7d29-8f83-515bbc062e25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e172a-6e74-7d29-8f83-515bbc062e25" xlink:to="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_FastenersMember_019e172a-6e74-7871-b849-28c7bdb7e8fe" xlink:href="fast-20260630.xsd#fast_FastenersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_FastenersMember_019e172a-6e74-7871-b849-28c7bdb7e8fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_ToolsMember_019e172a-6e74-7dc6-96d1-fb0c5eeef543" xlink:href="fast-20260630.xsd#fast_ToolsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_ToolsMember_019e172a-6e74-7dc6-96d1-fb0c5eeef543" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_CuttingToolsMember_019e172a-6e74-7b03-8f0c-44e94f19844a" xlink:href="fast-20260630.xsd#fast_CuttingToolsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_CuttingToolsMember_019e172a-6e74-7b03-8f0c-44e94f19844a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_HydraulicsAndPneumaticsMember_019e172a-6e74-7320-8471-e7ce4b67a6ca" xlink:href="fast-20260630.xsd#fast_HydraulicsAndPneumaticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_HydraulicsAndPneumaticsMember_019e172a-6e74-7320-8471-e7ce4b67a6ca" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_MaterialHandlingMember_019e172a-6e74-7d0c-a095-e915677eb40b" xlink:href="fast-20260630.xsd#fast_MaterialHandlingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_MaterialHandlingMember_019e172a-6e74-7d0c-a095-e915677eb40b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanitorialSuppliesMember_019e172a-6e74-7c4e-aa45-6f4aa6f5fd75" xlink:href="fast-20260630.xsd#fast_JanitorialSuppliesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_JanitorialSuppliesMember_019e172a-6e74-7c4e-aa45-6f4aa6f5fd75" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_ElectricalSuppliesMember_019e172a-6e74-72ac-8c07-6e8f863145bf" xlink:href="fast-20260630.xsd#fast_ElectricalSuppliesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_ElectricalSuppliesMember_019e172a-6e74-72ac-8c07-6e8f863145bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_WeldingSuppliesMember_019e172a-6e74-74bc-89ed-fc1c0392815f" xlink:href="fast-20260630.xsd#fast_WeldingSuppliesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_WeldingSuppliesMember_019e172a-6e74-74bc-89ed-fc1c0392815f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_SafetySuppliesMember_019e172a-6e74-7608-8e6d-ea3009c51546" xlink:href="fast-20260630.xsd#fast_SafetySuppliesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_fast_SafetySuppliesMember_019e172a-6e74-7608-8e6d-ea3009c51546" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductAndServiceOtherMember_019e172a-6e74-7fde-8ccd-e844699c6a40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProductAndServiceOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e172a-6e74-7393-8d2b-dab8b1bdbf4a" xlink:to="loc_us-gaap_ProductAndServiceOtherMember_019e172a-6e74-7fde-8ccd-e844699c6a40" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-783c-8227-ba193c4e8897" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019e172a-6e74-7b71-944a-500d84fb1efb" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-783c-8227-ba193c4e8897" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-783c-8227-ba193c4e8897_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-783c-8227-ba193c4e8897" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-783c-8227-ba193c4e8897_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7ce7-b500-ea026195b031" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e172a-6e74-783c-8227-ba193c4e8897" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7ce7-b500-ea026195b031" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductConcentrationRiskMember_019e172a-6e74-7df1-b65e-08edf26a35e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProductConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e172a-6e74-7ce7-b500-ea026195b031" xlink:to="loc_us-gaap_ProductConcentrationRiskMember_019e172a-6e74-7df1-b65e-08edf26a35e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7d4c-9b1e-93a8d8758e6a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019e172a-6e74-7b71-944a-500d84fb1efb" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7d4c-9b1e-93a8d8758e6a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7d4c-9b1e-93a8d8758e6a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7d4c-9b1e-93a8d8758e6a" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-7d4c-9b1e-93a8d8758e6a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-751c-b97d-a34a5939c474" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e172a-6e74-7d4c-9b1e-93a8d8758e6a" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-751c-b97d-a34a5939c474" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_019e172a-6e74-72d6-a9ff-1cd7d7d4bada" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e172a-6e74-751c-b97d-a34a5939c474" xlink:to="loc_us-gaap_SalesRevenueNetMember_019e172a-6e74-72d6-a9ff-1cd7d7d4bada" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019e172a-6e74-7178-9bc4-c6a48877ef36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e172a-6e74-7db2-8a34-6ac20a7da4ae" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019e172a-6e74-7178-9bc4-c6a48877ef36" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/StockholdersEquityAdditionalInformationDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#StockholdersEquityAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/StockholdersEquityAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019e172a-6e74-7773-b886-f2968aeacd2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019e172a-6e74-7351-b42a-6402729721e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019e172a-6e74-7773-b886-f2968aeacd2d" xlink:to="loc_us-gaap_SubsequentEventTable_019e172a-6e74-7351-b42a-6402729721e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019e172a-6e74-78f3-82cc-56e0e0f9955b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019e172a-6e74-7351-b42a-6402729721e1" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019e172a-6e74-78f3-82cc-56e0e0f9955b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019e172a-6e74-78f3-82cc-56e0e0f9955b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019e172a-6e74-78f3-82cc-56e0e0f9955b" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019e172a-6e74-78f3-82cc-56e0e0f9955b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019e172a-6e74-7856-9b50-38f755f7c089" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019e172a-6e74-78f3-82cc-56e0e0f9955b" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019e172a-6e74-7856-9b50-38f755f7c089" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019e172a-6e74-7120-930f-3a8034036397" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019e172a-6e74-7856-9b50-38f755f7c089" xlink:to="loc_us-gaap_SubsequentEventMember_019e172a-6e74-7120-930f-3a8034036397" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_019e172a-6e74-7115-ba5c-a5e7eeb28ebe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019e172a-6e74-7773-b886-f2968aeacd2d" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_019e172a-6e74-7115-ba5c-a5e7eeb28ebe" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/StockholdersEquityScheduleofDividendsPaidPreviouslyorDeclaredDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#StockholdersEquityScheduleofDividendsPaidPreviouslyorDeclaredDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/StockholdersEquityScheduleofDividendsPaidPreviouslyorDeclaredDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019e172a-6e74-72aa-9f30-6d63aadc530d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019e172a-6e74-7760-8989-6138a62d1f34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019e172a-6e74-72aa-9f30-6d63aadc530d" xlink:to="loc_us-gaap_SubsequentEventTable_019e172a-6e74-7760-8989-6138a62d1f34" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019e172a-6e74-7afd-bc8e-9e517d8814a8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019e172a-6e74-7760-8989-6138a62d1f34" xlink:to="loc_srt_StatementScenarioAxis_019e172a-6e74-7afd-bc8e-9e517d8814a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019e172a-6e74-7afd-bc8e-9e517d8814a8_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019e172a-6e74-7afd-bc8e-9e517d8814a8" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019e172a-6e74-7afd-bc8e-9e517d8814a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019e172a-6e74-7050-9fa9-5f91a8982e3c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019e172a-6e74-7afd-bc8e-9e517d8814a8" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019e172a-6e74-7050-9fa9-5f91a8982e3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_019e172a-6e74-7433-82f9-74fc10c066d5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_019e172a-6e74-7050-9fa9-5f91a8982e3c" xlink:to="loc_srt_ScenarioForecastMember_019e172a-6e74-7433-82f9-74fc10c066d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareCashPaid_019e172a-6e74-715b-a5b6-e4b2c4e157d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockDividendsPerShareCashPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019e172a-6e74-72aa-9f30-6d63aadc530d" xlink:to="loc_us-gaap_CommonStockDividendsPerShareCashPaid_019e172a-6e74-715b-a5b6-e4b2c4e157d4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/StockholdersEquityStockOptionsGrantedDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#StockholdersEquityStockOptionsGrantedDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/StockholdersEquityStockOptionsGrantedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e172a-6e74-7cb7-88de-e4e53b5dbf32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e172a-6e74-7cb7-88de-e4e53b5dbf32" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_019e172a-6e74-7d0c-9204-a37150076a93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e172a-6e74-7cb7-88de-e4e53b5dbf32" xlink:to="loc_us-gaap_AwardDateAxis_019e172a-6e74-7d0c-9204-a37150076a93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_019e172a-6e74-7d0c-9204-a37150076a93_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_019e172a-6e74-7d0c-9204-a37150076a93" xlink:to="loc_us-gaap_AwardDateDomain_019e172a-6e74-7d0c-9204-a37150076a93_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_019e172a-6e74-7d0c-9204-a37150076a93" xlink:to="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentySixMember_019e172a-6e74-7365-9569-cedf453d8118" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentySixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuarySecondTwoThousandTwentySixMember_019e172a-6e74-7365-9569-cedf453d8118" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentyFiveMember_019e172a-6e74-7b49-b20b-fc5b4bba6ae4" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentyFiveMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuarySecondTwoThousandTwentyFiveMember_019e172a-6e74-7b49-b20b-fc5b4bba6ae4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentyFourMember_019e172a-6e74-727c-b459-8b0a7f91d758" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuarySecondTwoThousandTwentyFourMember_019e172a-6e74-727c-b459-8b0a7f91d758" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryThirdTwoThousandTwentyThreeMember_019e172a-6e74-71de-affd-dacb347676d9" xlink:href="fast-20260630.xsd#fast_JanuaryThirdTwoThousandTwentyThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuaryThirdTwoThousandTwentyThreeMember_019e172a-6e74-71de-affd-dacb347676d9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryThirdTwoThousandTwentyTwoMember_019e172a-6e74-7ed5-83ac-a1d855403aa7" xlink:href="fast-20260630.xsd#fast_JanuaryThirdTwoThousandTwentyTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuaryThirdTwoThousandTwentyTwoMember_019e172a-6e74-7ed5-83ac-a1d855403aa7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryFourthTwoThousandTwentyOneMember_019e172a-6e74-7c0a-a30d-bf2129c498ee" xlink:href="fast-20260630.xsd#fast_JanuaryFourthTwoThousandTwentyOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuaryFourthTwoThousandTwentyOneMember_019e172a-6e74-7c0a-a30d-bf2129c498ee" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentyMember_019e172a-6e74-7590-b3ce-26621e3a9a63" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuarySecondTwoThousandTwentyMember_019e172a-6e74-7590-b3ce-26621e3a9a63" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandNineteenMember_019e172a-6e74-7e1e-9cfe-85247e5caedc" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandNineteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuarySecondTwoThousandNineteenMember_019e172a-6e74-7e1e-9cfe-85247e5caedc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandEighteenMember_019e172a-6e74-7f3a-84b1-ff961d11944e" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandEighteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuarySecondTwoThousandEighteenMember_019e172a-6e74-7f3a-84b1-ff961d11944e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryThirdTwoThousandSeventeenMember_019e172a-6e74-7d9f-90d8-29730aa73edb" xlink:href="fast-20260630.xsd#fast_JanuaryThirdTwoThousandSeventeenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-7746-a5d6-8dacd16ec2e3" xlink:to="loc_fast_JanuaryThirdTwoThousandSeventeenMember_019e172a-6e74-7d9f-90d8-29730aa73edb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_019e172a-6e74-76b8-816a-eb9eb03fa759" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_019e172a-6e74-76b8-816a-eb9eb03fa759" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e172a-6e74-7d22-a701-ad293c37b070" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e172a-6e74-7d22-a701-ad293c37b070" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_019e172a-6e74-739f-bc59-75ccd1a890e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:to="loc_us-gaap_SharePrice_019e172a-6e74-739f-bc59-75ccd1a890e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e172a-6e74-718c-98a1-470beb061c3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e172a-6e74-718c-98a1-470beb061c3e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e172a-6e74-7f80-9fdd-369cfc24e78f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-76d6-8337-ed2611b36c0e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e172a-6e74-7f80-9fdd-369cfc24e78f" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/StockholdersEquityFairValueAssumptionsforOptionsGrantedDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#StockholdersEquityFairValueAssumptionsforOptionsGrantedDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/StockholdersEquityFairValueAssumptionsforOptionsGrantedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e172a-6e74-76ae-9b22-ba6a322b3178" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e172a-6e74-76ae-9b22-ba6a322b3178" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_019e172a-6e74-7b25-bd7c-66546df21cd3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e172a-6e74-76ae-9b22-ba6a322b3178" xlink:to="loc_us-gaap_AwardDateAxis_019e172a-6e74-7b25-bd7c-66546df21cd3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_019e172a-6e74-7b25-bd7c-66546df21cd3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_019e172a-6e74-7b25-bd7c-66546df21cd3" xlink:to="loc_us-gaap_AwardDateDomain_019e172a-6e74-7b25-bd7c-66546df21cd3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_019e172a-6e74-7b25-bd7c-66546df21cd3" xlink:to="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentySixMember_019e172a-6e74-78e8-83be-2a301da5af11" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentySixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuarySecondTwoThousandTwentySixMember_019e172a-6e74-78e8-83be-2a301da5af11" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentyFiveMember_019e172a-6e74-74dd-9e4f-3e7b4b743625" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentyFiveMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuarySecondTwoThousandTwentyFiveMember_019e172a-6e74-74dd-9e4f-3e7b4b743625" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentyFourMember_019e172a-6e74-799b-b638-d4d73b93dc7d" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuarySecondTwoThousandTwentyFourMember_019e172a-6e74-799b-b638-d4d73b93dc7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryThirdTwoThousandTwentyThreeMember_019e172a-6e74-7a7d-8d51-32a5662211a2" xlink:href="fast-20260630.xsd#fast_JanuaryThirdTwoThousandTwentyThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuaryThirdTwoThousandTwentyThreeMember_019e172a-6e74-7a7d-8d51-32a5662211a2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryThirdTwoThousandTwentyTwoMember_019e172a-6e74-7a41-a6b0-617ed9e5e388" xlink:href="fast-20260630.xsd#fast_JanuaryThirdTwoThousandTwentyTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuaryThirdTwoThousandTwentyTwoMember_019e172a-6e74-7a41-a6b0-617ed9e5e388" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryFourthTwoThousandTwentyOneMember_019e172a-6e74-71de-a524-5d08c43572d1" xlink:href="fast-20260630.xsd#fast_JanuaryFourthTwoThousandTwentyOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuaryFourthTwoThousandTwentyOneMember_019e172a-6e74-71de-a524-5d08c43572d1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandTwentyMember_019e172a-6e74-7084-88aa-0482a396d863" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandTwentyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuarySecondTwoThousandTwentyMember_019e172a-6e74-7084-88aa-0482a396d863" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandNineteenMember_019e172a-6e74-72a3-b469-bcc54edb9680" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandNineteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuarySecondTwoThousandNineteenMember_019e172a-6e74-72a3-b469-bcc54edb9680" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuarySecondTwoThousandEighteenMember_019e172a-6e74-7d1d-9dec-0fc13a8ec53b" xlink:href="fast-20260630.xsd#fast_JanuarySecondTwoThousandEighteenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuarySecondTwoThousandEighteenMember_019e172a-6e74-7d1d-9dec-0fc13a8ec53b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_JanuaryThirdTwoThousandSeventeenMember_019e172a-6e74-79f5-965b-e6ea12d9b7c0" xlink:href="fast-20260630.xsd#fast_JanuaryThirdTwoThousandSeventeenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e172a-6e74-77d5-86e1-361f929db2e7" xlink:to="loc_fast_JanuaryThirdTwoThousandSeventeenMember_019e172a-6e74-79f5-965b-e6ea12d9b7c0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e172a-6e74-77e5-b578-588ff988f763" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e172a-6e74-77e5-b578-588ff988f763" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e172a-6e74-70f1-a41a-e9b7757e889c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e172a-6e74-70f1-a41a-e9b7757e889c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e172a-6e74-7aec-87b8-6eeb248ea09b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e172a-6e74-7aec-87b8-6eeb248ea09b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e172a-6e74-7137-90b5-59a7681b71d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e172a-6e74-7137-90b5-59a7681b71d0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e172a-6e74-741b-b38c-b978691124d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e172a-6e74-7158-b55f-da205563980e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e172a-6e74-741b-b38c-b978691124d7" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/DebtCommitmentsDebtObligationsandLettersofCreditOutstandingDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#DebtCommitmentsDebtObligationsandLettersofCreditOutstandingDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/DebtCommitmentsDebtObligationsandLettersofCreditOutstandingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7bba-81f4-aec76e6c8237" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7bba-81f4-aec76e6c8237" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7691-b680-985f515fa463" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7bba-81f4-aec76e6c8237" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7691-b680-985f515fa463" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7691-b680-985f515fa463_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7691-b680-985f515fa463" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7691-b680-985f515fa463_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7583-8f88-1e999a434d4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7691-b680-985f515fa463" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7583-8f88-1e999a434d4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e172a-6e74-771b-ba7a-22085ff56b00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7583-8f88-1e999a434d4d" xlink:to="loc_us-gaap_LineOfCreditMember_019e172a-6e74-771b-ba7a-22085ff56b00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019e172a-6e74-766e-9a8e-85f8aa22debf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7583-8f88-1e999a434d4d" xlink:to="loc_us-gaap_SeniorNotesMember_019e172a-6e74-766e-9a8e-85f8aa22debf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e172a-6e74-7b09-9168-2afa0fe6d985" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7bba-81f4-aec76e6c8237" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e172a-6e74-7b09-9168-2afa0fe6d985" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-7b09-9168-2afa0fe6d985_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e172a-6e74-7b09-9168-2afa0fe6d985" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-7b09-9168-2afa0fe6d985_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-756e-931d-6070fa56e1da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e172a-6e74-7b09-9168-2afa0fe6d985" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-756e-931d-6070fa56e1da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_A2.72SeniorUnsecuredPromissoryNotePayableMember_019e172a-6e74-7a89-86c0-3e95cb965891" xlink:href="fast-20260630.xsd#fast_A2.72SeniorUnsecuredPromissoryNotePayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-756e-931d-6070fa56e1da" xlink:to="loc_fast_A2.72SeniorUnsecuredPromissoryNotePayableMember_019e172a-6e74-7a89-86c0-3e95cb965891" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_A2.13SeniorUnsecuredPromissoryNotePayableMember_019e172a-6e74-7380-9b83-1c056a4fb0db" xlink:href="fast-20260630.xsd#fast_A2.13SeniorUnsecuredPromissoryNotePayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-756e-931d-6070fa56e1da" xlink:to="loc_fast_A2.13SeniorUnsecuredPromissoryNotePayableMember_019e172a-6e74-7380-9b83-1c056a4fb0db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_A2.50SeniorUnsecuredPromissoryNotePayableMember_019e172a-6e74-7b4b-9eb9-9278f6e4a8da" xlink:href="fast-20260630.xsd#fast_A2.50SeniorUnsecuredPromissoryNotePayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e172a-6e74-756e-931d-6070fa56e1da" xlink:to="loc_fast_A2.50SeniorUnsecuredPromissoryNotePayableMember_019e172a-6e74-7b4b-9eb9-9278f6e4a8da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-76dd-800b-59d6621eeda3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7bba-81f4-aec76e6c8237" xlink:to="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-76dd-800b-59d6621eeda3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-76dd-800b-59d6621eeda3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-76dd-800b-59d6621eeda3" xlink:to="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-76dd-800b-59d6621eeda3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-71d5-8bf3-2449fb1bdd8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-76dd-800b-59d6621eeda3" xlink:to="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-71d5-8bf3-2449fb1bdd8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e172a-6e74-7ea5-b921-cd6bb33043f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-71d5-8bf3-2449fb1bdd8b" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e172a-6e74-7ea5-b921-cd6bb33043f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_019e172a-6e74-72aa-80c6-224edea0ca64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevolvingCreditFacilityMember_019e172a-6e74-7ea5-b921-cd6bb33043f7" xlink:to="loc_us-gaap_LetterOfCreditMember_019e172a-6e74-72aa-80c6-224edea0ca64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e172a-6e74-70f8-94e4-a6244bd449e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e172a-6e74-70f8-94e4-a6244bd449e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e172a-6e74-7ead-a856-2efb508dfab8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e172a-6e74-7ead-a856-2efb508dfab8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e172a-6e74-73e0-982e-abc6eaa72189" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_LongTermDebt_019e172a-6e74-73e0-982e-abc6eaa72189" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019e172a-6e74-7c97-95d1-e072ad8cbc4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_LongTermDebtCurrent_019e172a-6e74-7c97-95d1-e072ad8cbc4b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e172a-6e74-7f6e-8afc-ce43feb87fb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e172a-6e74-7f6e-8afc-ce43feb87fb0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019e172a-6e74-715e-9382-f2b27daa0131" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7857-9f42-5c36fb7f7763" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019e172a-6e74-715e-9382-f2b27daa0131" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/DebtCommitmentsUnsecuredRevolvingCreditFacilityDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#DebtCommitmentsUnsecuredRevolvingCreditFacilityDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/DebtCommitmentsUnsecuredRevolvingCreditFacilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-774c-9563-349ec9e8d426" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-774c-9563-349ec9e8d426" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7ebf-a409-44e539a14a80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-774c-9563-349ec9e8d426" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7ebf-a409-44e539a14a80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7ebf-a409-44e539a14a80_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7ebf-a409-44e539a14a80" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-7ebf-a409-44e539a14a80_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-74e9-a04a-516d0531b694" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-7ebf-a409-44e539a14a80" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-74e9-a04a-516d0531b694" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e172a-6e74-7559-93de-886f2d134d40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-74e9-a04a-516d0531b694" xlink:to="loc_us-gaap_LineOfCreditMember_019e172a-6e74-7559-93de-886f2d134d40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-772b-8cfb-759cf748b195" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-774c-9563-349ec9e8d426" xlink:to="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-772b-8cfb-759cf748b195" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-772b-8cfb-759cf748b195_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-772b-8cfb-759cf748b195" xlink:to="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-772b-8cfb-759cf748b195_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-7e55-9c98-74f23093c4c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e172a-6e74-772b-8cfb-759cf748b195" xlink:to="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-7e55-9c98-74f23093c4c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e172a-6e74-7674-a251-d2355b2a34cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-7e55-9c98-74f23093c4c0" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e172a-6e74-7674-a251-d2355b2a34cb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_019e172a-6e74-718e-ab8d-af7995e9d9ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e172a-6e74-7e55-9c98-74f23093c4c0" xlink:to="loc_us-gaap_LetterOfCreditMember_019e172a-6e74-718e-ab8d-af7995e9d9ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e172a-6e74-78e9-91f5-607bd0da0284" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-774c-9563-349ec9e8d426" xlink:to="loc_srt_RangeAxis_019e172a-6e74-78e9-91f5-607bd0da0284" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e172a-6e74-78e9-91f5-607bd0da0284_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e172a-6e74-78e9-91f5-607bd0da0284" xlink:to="loc_srt_RangeMember_019e172a-6e74-78e9-91f5-607bd0da0284_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e172a-6e74-714b-ae18-269a47eed6a1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e172a-6e74-78e9-91f5-607bd0da0284" xlink:to="loc_srt_RangeMember_019e172a-6e74-714b-ae18-269a47eed6a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e172a-6e74-71b6-aed1-53dd1b13a698" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e172a-6e74-714b-ae18-269a47eed6a1" xlink:to="loc_srt_MinimumMember_019e172a-6e74-71b6-aed1-53dd1b13a698" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e172a-6e74-76fb-8b35-8f8d079ec30f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e172a-6e74-714b-ae18-269a47eed6a1" xlink:to="loc_srt_MaximumMember_019e172a-6e74-76fb-8b35-8f8d079ec30f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e172a-6e74-7443-8006-d511e6add8e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e172a-6e74-7443-8006-d511e6add8e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_LineOfCreditFacilityUncommittedAccordionOptionIncreaseLimit_019e172a-6e74-7d3f-9a56-df05fdb73687" xlink:href="fast-20260630.xsd#fast_LineOfCreditFacilityUncommittedAccordionOptionIncreaseLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_fast_LineOfCreditFacilityUncommittedAccordionOptionIncreaseLimit_019e172a-6e74-7d3f-9a56-df05fdb73687" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_LineOfCreditFacilityUncommittedAccordionOptionHigherBorrowingCapacityOption_019e172a-6e74-7e8c-bf7a-7481a3b0fb51" xlink:href="fast-20260630.xsd#fast_LineOfCreditFacilityUncommittedAccordionOptionHigherBorrowingCapacityOption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_fast_LineOfCreditFacilityUncommittedAccordionOptionHigherBorrowingCapacityOption_019e172a-6e74-7e8c-bf7a-7481a3b0fb51" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_DebtInstrumentBasisSpreadOnVariableRateSpreadAdjustment_019e172a-6e74-7c97-9b38-b767517912f8" xlink:href="fast-20260630.xsd#fast_DebtInstrumentBasisSpreadOnVariableRateSpreadAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_fast_DebtInstrumentBasisSpreadOnVariableRateSpreadAdjustment_019e172a-6e74-7c97-9b38-b767517912f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e172a-6e74-7b0e-a7b0-78f78998243e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e172a-6e74-7b0e-a7b0-78f78998243e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e172a-6e74-728b-acb9-a4dd33bc423d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-71bb-9029-602082ca3bdb" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e172a-6e74-728b-acb9-a4dd33bc423d" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/DebtCommitmentsSeniorUnsecuredPromissoryNotesPayableDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#DebtCommitmentsSeniorUnsecuredPromissoryNotesPayableDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/DebtCommitmentsSeniorUnsecuredPromissoryNotesPayableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7239-89ba-039d1e1c9fce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7622-980a-cf132afa778b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7239-89ba-039d1e1c9fce" xlink:to="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7622-980a-cf132afa778b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-732c-94fc-029b9b1b6459" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e172a-6e74-7622-980a-cf132afa778b" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-732c-94fc-029b9b1b6459" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-732c-94fc-029b9b1b6459_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-732c-94fc-029b9b1b6459" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-732c-94fc-029b9b1b6459_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-73c0-b12f-4c408b57e658" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e172a-6e74-732c-94fc-029b9b1b6459" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-73c0-b12f-4c408b57e658" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019e172a-6e74-767a-ab1b-8c81f6347c77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e172a-6e74-73c0-b12f-4c408b57e658" xlink:to="loc_us-gaap_SeniorNotesMember_019e172a-6e74-767a-ab1b-8c81f6347c77" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e172a-6e74-73e1-8a31-9088653a80c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7239-89ba-039d1e1c9fce" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e172a-6e74-73e1-8a31-9088653a80c8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_DebtInstrumentMaximumFaceAmountAllowable_019e172a-6e74-7e74-8ddb-819fff7dec17" xlink:href="fast-20260630.xsd#fast_DebtInstrumentMaximumFaceAmountAllowable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e172a-6e74-7239-89ba-039d1e1c9fce" xlink:to="loc_fast_DebtInstrumentMaximumFaceAmountAllowable_019e172a-6e74-7e74-8ddb-819fff7dec17" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/SegmentReportingReportableSegmentNetSalesfromExternalCustomersDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#SegmentReportingReportableSegmentNetSalesfromExternalCustomersDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/SegmentReportingReportableSegmentNetSalesfromExternalCustomersDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7054-ad96-0e6ab76d172a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7f03-b42a-6a47d68c8088" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7054-ad96-0e6ab76d172a" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7f03-b42a-6a47d68c8088" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7398-bedb-0f002699fbec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7f03-b42a-6a47d68c8088" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7398-bedb-0f002699fbec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-7398-bedb-0f002699fbec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7398-bedb-0f002699fbec" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-7398-bedb-0f002699fbec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-740a-865b-26e49d507b03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7398-bedb-0f002699fbec" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-740a-865b-26e49d507b03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-7cf0-bfb5-1126b2701f35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e172a-6e74-740a-865b-26e49d507b03" xlink:to="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-7cf0-bfb5-1126b2701f35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e172a-6e74-7e83-b607-4c8f007cf17a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7054-ad96-0e6ab76d172a" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e172a-6e74-7e83-b607-4c8f007cf17a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/SegmentReportingReconciliationofReportableSegmentROAPreTaxProfittoConsolidatedIncomebeforeIncomeTaxesDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#SegmentReportingReconciliationofReportableSegmentROAPreTaxProfittoConsolidatedIncomebeforeIncomeTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/SegmentReportingReconciliationofReportableSegmentROAPreTaxProfittoConsolidatedIncomebeforeIncomeTaxesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7efa-85b7-7d8baa0c35e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7f7f-bf6a-22c533c7d139" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7efa-85b7-7d8baa0c35e5" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7f7f-bf6a-22c533c7d139" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7bdb-b488-d697542e57cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7f7f-bf6a-22c533c7d139" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7bdb-b488-d697542e57cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-7bdb-b488-d697542e57cb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7bdb-b488-d697542e57cb" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-7bdb-b488-d697542e57cb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-7a7a-88e6-c078f37a818f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7bdb-b488-d697542e57cb" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-7a7a-88e6-c078f37a818f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-7363-8a06-19413ab75ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e172a-6e74-7a7a-88e6-c078f37a818f" xlink:to="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-7363-8a06-19413ab75ac9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_OtherOperatingSegmentMember_019e172a-6e74-7ff5-8fca-70727be06159" xlink:href="fast-20260630.xsd#fast_OtherOperatingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e172a-6e74-7a7a-88e6-c078f37a818f" xlink:to="loc_fast_OtherOperatingSegmentMember_019e172a-6e74-7ff5-8fca-70727be06159" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_SegmentReportingSegmentItemROAPreTaxProfit_019e172a-6e74-79b8-88a1-81cbd2cdb08a" xlink:href="fast-20260630.xsd#fast_SegmentReportingSegmentItemROAPreTaxProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7efa-85b7-7d8baa0c35e5" xlink:to="loc_fast_SegmentReportingSegmentItemROAPreTaxProfit_019e172a-6e74-79b8-88a1-81cbd2cdb08a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_SegmentReportingOtherOperatingSegmentsItemROAPreTaxProfit_019e172a-6e74-7dd9-b377-14256f298868" xlink:href="fast-20260630.xsd#fast_SegmentReportingOtherOperatingSegmentsItemROAPreTaxProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7efa-85b7-7d8baa0c35e5" xlink:to="loc_fast_SegmentReportingOtherOperatingSegmentsItemROAPreTaxProfit_019e172a-6e74-7dd9-b377-14256f298868" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e172a-6e74-708c-8592-a918aa4c24b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-7efa-85b7-7d8baa0c35e5" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e172a-6e74-708c-8592-a918aa4c24b7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/SegmentReportingReportableSegmentROAAssetsDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#SegmentReportingReportableSegmentROAAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/SegmentReportingReportableSegmentROAAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_fast_ScheduleOfSegmentReportingInformationBySegmentLineItems_019e172a-6e74-7b94-bb85-b8dd4f28bc7f" xlink:href="fast-20260630.xsd#fast_ScheduleOfSegmentReportingInformationBySegmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7321-94bd-cf1764d7efc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_fast_ScheduleOfSegmentReportingInformationBySegmentLineItems_019e172a-6e74-7b94-bb85-b8dd4f28bc7f" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7321-94bd-cf1764d7efc8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7587-871a-8343d4897a31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7321-94bd-cf1764d7efc8" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7587-871a-8343d4897a31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-7587-871a-8343d4897a31_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7587-871a-8343d4897a31" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-7587-871a-8343d4897a31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-7cc5-96ac-9f96ec52101f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-7587-871a-8343d4897a31" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-7cc5-96ac-9f96ec52101f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-72a1-9604-624efdbdc1f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e172a-6e74-7cc5-96ac-9f96ec52101f" xlink:to="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-72a1-9604-624efdbdc1f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_SegmentReportingSegmentItemROAAssets_019e172a-6e74-7558-bd41-1b603e84fa3e" xlink:href="fast-20260630.xsd#fast_SegmentReportingSegmentItemROAAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fast_ScheduleOfSegmentReportingInformationBySegmentLineItems_019e172a-6e74-7b94-bb85-b8dd4f28bc7f" xlink:to="loc_fast_SegmentReportingSegmentItemROAAssets_019e172a-6e74-7558-bd41-1b603e84fa3e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fastenal.com/role/SegmentReportingOtherSegmentDisclosuresDetails" xlink:type="simple" xlink:href="fast-20260630.xsd#SegmentReportingOtherSegmentDisclosuresDetails"/>
  <link:definitionLink xlink:role="http://www.fastenal.com/role/SegmentReportingOtherSegmentDisclosuresDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-70c0-918f-230ab96a4a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7731-aba1-3289bb02181d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-70c0-918f-230ab96a4a5c" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7731-aba1-3289bb02181d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-74cf-b1f9-b9de3fe6b2e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e172a-6e74-7731-aba1-3289bb02181d" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-74cf-b1f9-b9de3fe6b2e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-74cf-b1f9-b9de3fe6b2e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-74cf-b1f9-b9de3fe6b2e9" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-74cf-b1f9-b9de3fe6b2e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e172a-6e74-7ef4-8b4e-12ecbc987c25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e172a-6e74-74cf-b1f9-b9de3fe6b2e9" xlink:to="loc_us-gaap_SegmentDomain_019e172a-6e74-7ef4-8b4e-12ecbc987c25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-7bba-926c-87ac5f7edf6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e172a-6e74-7ef4-8b4e-12ecbc987c25" xlink:to="loc_us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember_019e172a-6e74-7bba-926c-87ac5f7edf6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fast_SegmentReportingSegmentItemROAPreTaxProfitDepreciationAndAmortizationExpense_019e172a-6e74-783a-9151-3d92a4941d55" xlink:href="fast-20260630.xsd#fast_SegmentReportingSegmentItemROAPreTaxProfitDepreciationAndAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e172a-6e74-70c0-918f-230ab96a4a5c" xlink:to="loc_fast_SegmentReportingSegmentItemROAPreTaxProfitDepreciationAndAmortizationExpense_019e172a-6e74-783a-9151-3d92a4941d55" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
