<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-01-25</periodOfReport>

    <issuer>
        <issuerCik>0000846901</issuerCik>
        <issuerName>LAKELAND BANCORP INC</issuerName>
        <issuerTradingSymbol>LBAI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001098513</rptOwnerCik>
            <rptOwnerName>GUPTILL GEORGE H JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O LAKELAND BANCORP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>250 OAK RIDGE ROAD</rptOwnerStreet2>
            <rptOwnerCity>OAK RIDGE</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07438</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-01-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1198</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.43</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>83282</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">D: Shares issued by Company as payment of annual retainer as a director of the Company.  I: 623,549 shares held by Franklin Mutual Insurance Company of which Mr. Guptill is Chairman.  In Mr. Guptill's capacity of Chairman of Franklin Mutual Insurance Company, he does not vote, invest, or dispose of these shares.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Harry Cooper POA</signatureName>
        <signatureDate>2011-01-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
