<SEC-DOCUMENT>0000352825-25-000036.txt : 20250224
<SEC-HEADER>0000352825-25-000036.hdr.sgml : 20250224
<ACCEPTANCE-DATETIME>20250224172718
ACCESSION NUMBER:		0000352825-25-000036
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20250220
FILED AS OF DATE:		20250224
DATE AS OF CHANGE:		20250224

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:		25658297

	MAIL ADDRESS:	
		STREET 1:		TOLLGRADE COMMUNICATIONS INC
		STREET 2:		493 NIXON RD.
		CITY:			CHESWICK
		STATE:			PA
		ZIP:			15024

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
		IRS NUMBER:				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_1740436030.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-02-20</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>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-02-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1595</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23075</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-02-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3063</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26138</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-02-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>859</value>
                    <footnoteId id="F6"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26997</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F7"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2025-02-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1614</value>
                    <footnoteId id="F8"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.65</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>25383</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F7"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>806</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 2022-2024 Long Term Incentive Plan granted on 2/17/22 upon certification of performance results by the Compensation Committee at 35.7% for the annual period ended December 31, 2024.</footnote>
        <footnote id="F2">Includes 3,725 shares of common stock resulting from the settlement of Performance Restricted Stock Units earned under the 2022-2024 Long Term Incentive Plan granted on 02/17/2022; those 3,725 Performance Restricted Stock Units settled at the end of the 2022-2024 performance period upon certification by the Compensation Committee on 2/20/2025.</footnote>
        <footnote id="F3">Includes 3,551 Performance Restricted Stock Units earned under the 2023-2025 Long Term Incentive Plan granted on 2/14/2023; those 3,551 Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2025, upon certification by the Compensation Committee.</footnote>
        <footnote id="F4">Represents the number of Performance Restricted Stock 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 48.4% for the annual period ended December 31, 2024.  The earned Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2025, upon certification by the Compensation Committee.</footnote>
        <footnote id="F5">Includes 6,614 Performance Restricted Stock Units earned under the 2023-2025 Long Term Incentive Plan granted on 2/14/2023; those 6,614 Performance Restricted Stock Units will settle at the end of the performance period on December 31, 2025, upon certification by the Compensation Committee.</footnote>
        <footnote id="F6">Represents the number of Performance Restricted Stock Units under the 2024-2026 Long Term Incentive Plan granted on 5/23/2024 upon certification of performance results by the Compensation Committee at 25.8% for the annual period ended December 31, 2024.  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="F7">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 of the Compensation Committee.</footnote>
        <footnote id="F8">Shares withheld to pay taxes upon the vesting of performance shares related to the 2022-2024 LTIP awarded on 2/17/22.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sean M. Reilly by Judith Balog, attorney-in-fact</signatureName>
        <signatureDate>2025-02-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
