-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 RkDYwqqJ3alEU3QQGbGkENdX3qFzDVURGa4R9zPwOpeIJImYZdo1yHCYX83KhFpi
 fM9I+qC8+JHMuywE+nEoTA==

<SEC-DOCUMENT>0001140361-09-013018.txt : 20090522
<SEC-HEADER>0001140361-09-013018.hdr.sgml : 20090522
<ACCEPTANCE-DATETIME>20090521150544
ACCESSION NUMBER:		0001140361-09-013018
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090521
FILED AS OF DATE:		20090521
DATE AS OF CHANGE:		20090521

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			OWEN DIANE B
		CENTRAL INDEX KEY:			0001215505

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-10436
		FILM NUMBER:		09845235

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
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-05-21</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001215505</rptOwnerCik>
            <rptOwnerName>OWEN DIANE B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O L.B. FOSTER COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>415 HOLIDAY DRIVE</rptOwnerStreet2>
            <rptOwnerCity>PITTSBURGH</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>15220</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2009-05-21</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1750</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>19046</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Option to Buy</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>7.81</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2004-05-25</value>
            </exerciseDate>
            <expirationDate>
                <value>2014-05-24</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Option to Buy</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>8.97</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2005-05-25</value>
            </exerciseDate>
            <expirationDate>
                <value>2015-05-24</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes></footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Diane B. Owen</signatureName>
        <signatureDate>2009-05-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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