<SEC-DOCUMENT>0001284955-26-000004.txt : 20260223
<SEC-HEADER>0001284955-26-000004.hdr.sgml : 20260223
<ACCEPTANCE-DATETIME>20260223161352
ACCESSION NUMBER:		0001284955-26-000004
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20260219
FILED AS OF DATE:		20260223
DATE AS OF CHANGE:		20260223

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			REILLY SEAN M
		CENTRAL INDEX KEY:			0001284955
		ORGANIZATION NAME:           	

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-10436
		FILM NUMBER:		26665308

	MAIL ADDRESS:	
		STREET 1:		C/O L.B. FOSTER COMPANY
		STREET 2:		415 HOLIDAY DRIVE, SUITE 100
		CITY:			PITTSBURGH
		STATE:			PA
		ZIP:			15220

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			FOSTER L B CO
		CENTRAL INDEX KEY:			0000352825
		STANDARD INDUSTRIAL CLASSIFICATION:	WHOLESALE-METALS SERVICE CENTERS & OFFICES [5051]
		ORGANIZATION NAME:           	07 Trade & Services
		EIN:				251324733
		STATE OF INCORPORATION:			PA
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		415 HOLIDAY DR
		CITY:			PITTSBURGH
		STATE:			PA
		ZIP:			15220
		BUSINESS PHONE:		4129283431

	MAIL ADDRESS:	
		STREET 1:		415 HOLIDAY DR
		CITY:			PITTSBURGH
		STATE:			PA
		ZIP:			15220
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wk-form4_1771881229.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
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    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-02-19</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000352825</issuerCik>
        <issuerName>FOSTER L B CO</issuerName>
        <issuerTradingSymbol>FSTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001284955</rptOwnerCik>
            <rptOwnerName>REILLY SEAN M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>L.B. FOSTER COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>415 HOLIDAY DRIVE, SUITE 100</rptOwnerStreet2>
            <rptOwnerCity>PITTSBURGH</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>15220</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Controller</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
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            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-02-19</value>
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            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>2987</value>
                    <footnoteId id="F1"/>
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                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>30005</value>
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        <nonDerivativeTransaction>
            <securityTitle>
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        <nonDerivativeTransaction>
            <securityTitle>
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                    <footnoteId id="F6"/>
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                    <value>31812</value>
                    <footnoteId id="F2"/>
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                    <footnoteId id="F7"/>
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                    <value>D</value>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-02-19</value>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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                    <value>2053</value>
                    <footnoteId id="F8"/>
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                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
                    <value>33865</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F7"/>
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                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-02-19</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>4308</value>
                    <footnoteId id="F9"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>31.125</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>29557</value>
                    <footnoteId id="F5"/>
                    <footnoteId id="F7"/>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1013</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>L.B. Foster Company 401(k) Plan Shares</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents the number of shares of common stock resulting from Performance Share Units earned under the 2023-2025 Long Term Incentive Plan granted on 2/14/2023 upon certification of performance results by the Compensation Committee at 47.2% for the annual period ended December 31, 2025.</footnote>
        <footnote id="F2">Includes 9,601 shares of common stock resulting from the settlement of Performance Restricted Stock Units earned under the 2023-2025 Long Term Incentive Plan granted on 2/14/2023; those 9,601 Performance Restricted Stock Units settled at the end of the 2023-2025 performance period upon certification by the Compensation Committee on 2/19/2026.</footnote>
        <footnote id="F3">Includes 859 Performance Restricted Stock Units earned under the 2024-2026 Long Term Incentive Plan granted on 5/23/2024; those 859 Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2026, upon certification by the Compensation Committee.</footnote>
        <footnote id="F4">Represents the number of Performance Restricted Stock Units earned under the 2024-2026 Long Term Incentive Plan granted on 5/23/2024 upon certification of performance results by the Compensation Committee at 39.5% for the annual period ended December 31, 2025.  The earned Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2026, upon certification by the Compensation Committee.</footnote>
        <footnote id="F5">Includes 2,174 Performance Restricted Stock Units earned under the 2024-2026 Long Term Incentive Plan granted on 5/23/2024; those 2,174 Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2026, upon certification of the Compensation Committee.</footnote>
        <footnote id="F6">Represents the number of Performance Restricted Stock Units earned under the 2025-2027 Long Term Incentive Plan granted on 5/22/2025 upon certification of performance results by the Compensation Committee at 11.2% for the annual period ended December 31, 2025.  The earned Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2027, upon certification by the Compensation Committee.</footnote>
        <footnote id="F7">Includes 492 Performance Restricted Stock Units earned under the 2025-2027 Long Term Incentive Plan granted on 5/22/2025; those 492 Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2027, upon certification by the Compensation Committee.</footnote>
        <footnote id="F8">Award of restricted stock units (RSUs), which are settled in stock upon vesting, and generally will vest ratably over a three-year period on the first, second, and third anniversary of the date of grant.</footnote>
        <footnote id="F9">Shares withheld to pay taxes upon the vesting and settlement of all earned performance shares related to the 2023-2025 LTIP awarded on 2/14/2023.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sean M. Reilly by Judith Balog, attorney-in-fact</signatureName>
        <signatureDate>2026-02-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
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</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
