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

    <schemaVersion>X0305</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-06-07</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000726517</issuerCik>
        <issuerName>MERCHANTS BANCSHARES INC</issuerName>
        <issuerTradingSymbol>MBVT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001049356</rptOwnerCik>
            <rptOwnerName>KANE JOHN A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MERCHANTS BANCSHARES, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>275 KENNEDY DRIVE</rptOwnerStreet2>
            <rptOwnerCity>SOUTH BURLINGTON</rptOwnerCity>
            <rptOwnerState>VT</rptOwnerState>
            <rptOwnerZipCode>05403</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>2012-06-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>26.738</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2053.888</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>800</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Family Trust</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares of Merchants Bancshares, Inc. common stock are held in this family trust for Mr. Kane's adult children, for whom Mr. Kane's wife is trustee. Mr. Kane disclaims beneficial ownership of the shares held in this trust.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Lisa A. Razo, P.O.A. for John A. Kane</signatureName>
        <signatureDate>2012-06-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0305
4
2012-06-07
0
0000726517
MERCHANTS BANCSHARES INC
MBVT
0001049356
KANE JOHN A
C/O MERCHANTS BANCSHARES, INC.
275 KENNEDY DRIVE
SOUTH BURLINGTON
VT
05403
1
0
0
0
Common Stock
2012-06-07
4
P
0
1000
26.738
A
2053.888
D
Common Stock
800
I
By Family Trust
Shares of Merchants Bancshares, Inc. common stock are held in this family trust for Mr. Kane's adult children, for whom Mr. Kane's wife is trustee. Mr. Kane disclaims beneficial ownership of the shares held in this trust.
/s/ Lisa A. Razo, P.O.A. for John A. Kane
2012-06-07