<?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>0001237150</rptOwnerCik>
            <rptOwnerName>NOONAN JAMES R</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>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP and Chief Credit Officer</officerTitle>
        </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>5568</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.43</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>19080</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Includes 5,012 shares of restricted stock that has not yet vested.</footnote>
        <footnote id="F2">Reference is made to shares of restricted stock awarded under the Lakeland Bancorp Inc. 2009 Equity Compensation Program (&quot;Plan&quot;).  Subject to vesting and transferability requirements and restrictions under the Emergency Economic Stabilization Act of 2008 (&quot;EESA&quot;), as amended by the American Recovery and Reinvestment Act of 2009 (&quot;ARRA&quot;), the Interim Final Rule promulgated by the U.S. Department of Treasury on June 15, 2009, as may be further amended, and the  Plan, the shares of the restricted stock shall vest as follows: 45% on January 25,2013, 45% on January 25, 2014, with the remaining 10% on the later of January 25, 2016 or the date on which the Company repays the Capital Purchase Program investment.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Harry Cooper POA</signatureName>
        <signatureDate>2011-01-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
