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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-04-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000713671</issuerCik>
        <issuerName>DNB FINANCIAL CORP /PA/</issuerName>
        <issuerTradingSymbol>DNBF</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001208605</rptOwnerCik>
            <rptOwnerName>LATOFF WILLIAM S</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1></rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity></rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode></rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman &amp; CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>DNB Common Stock,$1 par value, restricted stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>6.93</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-04-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>14200</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.93</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2013-04-23</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2013-04-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>DNB Common Stock,$1 par value</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>14200</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>72088</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted shares of common stock under the DNB Financial Corp.Incentive Equity &amp; Deferred Compensation Plan vesting on the ealier of 3 years from grant date, a change in control, death, termination of employment on account of disability, or 2 years from the Grant Date if he has continued to perform substantial services for DNB through that date, or the date he attains age 65. If employment terminates for any reason prior to the vesting date, the Grantee will forfeit all rights under the award. Grantee's entitlement to Award Shares shall be subject to the further conditions pursuant to Section 111(b)(3)(D) of EESA. During any period that the Grantee is subject to applicable limits on executive compensation under TARP, the Grantee's rights under the award agreement are only enforceable to the extent permitted under applicable TARP regulations.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Gerald F Sopp for William S Latoff</signatureName>
        <signatureDate>2010-05-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
