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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-03-05</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001220433</rptOwnerCik>
            <rptOwnerName>BOUYEA PETER A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>275 KENNEDY DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></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>2008-03-05</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>921</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.75</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>76855</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1317</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Spouse</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Mr. Bouyea disclaims beneficial ownership of shares of Merchants Bancshares, Inc. common stock held by his wife, as he does not control the voting or investment of these shares.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Lisa A. Razo, P.O.A. for Peter A. Bouyea</signatureName>
        <signatureDate>2008-03-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "primary_doc.xml", "description": "PRIMARY DOCUMENT", "secsgml_size_bytes": 3939}]
X0202
4
2008-03-05
0
0000726517
MERCHANTS BANCSHARES INC
mbvt
0001220433
BOUYEA PETER A
275 KENNEDY DRIVE
SOUTH BURLINGTON
VT
05403
1
0
0
0
Common Stock
2008-03-05
4
A
0
921
22.75
A
76855
D
Common Stock
1317
I
By Spouse
Mr. Bouyea disclaims beneficial ownership of shares of Merchants Bancshares, Inc. common stock held by his wife, as he does not control the voting or investment of these shares.
Lisa A. Razo, P.O.A. for Peter A. Bouyea
2008-03-07