<SEC-DOCUMENT>0001140361-16-054182.txt : 20160219
<SEC-HEADER>0001140361-16-054182.hdr.sgml : 20160219
<ACCEPTANCE-DATETIME>20160219161639
ACCESSION NUMBER:		0001140361-16-054182
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20160217
FILED AS OF DATE:		20160219
DATE AS OF CHANGE:		20160219

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			FOSTER L B CO
		CENTRAL INDEX KEY:			0000352825
		STANDARD INDUSTRIAL CLASSIFICATION:	WHOLESALE-METALS SERVICE CENTERS & OFFICES [5051]
		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

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			PAPAZOGLOU KONSTANTINOS
		CENTRAL INDEX KEY:			0001270977

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-10436
		FILM NUMBER:		161442058

	MAIL ADDRESS:	
		STREET 1:		900 OLD FREEPORT ROAD
		CITY:			PITTSBURGH
		STATE:			PA
		ZIP:			15238
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-02-17</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001270977</rptOwnerCik>
            <rptOwnerName>PAPAZOGLOU KONSTANTINOS</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>172 BRUNSWICK BLVD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>POINTE CLAIRE</rptOwnerCity>
            <rptOwnerState>A8</rptOwnerState>
            <rptOwnerZipCode>H9R 5P9</rptOwnerZipCode>
            <rptOwnerStateDescription>QUEBEC, CANADA</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-02-17</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2802</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12247</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Award of time vested restricted stock, which will vest in 33-1/3% increments on each of the first, second, and third anniversaries of the date of the grant.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Konstantinos Papazoglou by Amelia L. Beck, attorney-in-fact</signatureName>
        <signatureDate>2016-02-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
