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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-05-22</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>2008-05-22</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>22.6</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>80832</value>
                    <footnoteId id="F1"/>
                </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="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This Form 4 reflects the correction of an administrative error in reporting Mr. Bouyea's direct holdings of Merchants Bancshares, Inc. stock on a Form 4 filed on March 31, 2008.</footnote>
        <footnote id="F2">This Form 4 reflects the correction of an administrative error in omitting shares owned by Mr. Bouyea's wife on Forms 4 filed by Mr. Bouyea on March 7, 2008 through May 15, 2008. As reported on Merchants Bancshares, Inc.'s proxy statement dated April 1, 2008, Mr. Bouyea disclaims beneficial ownership of these shares, as he does not control the voting or investment of the shares.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Lisa A. Razo, P.O.A. for Peter A. Bouyea</signatureName>
        <signatureDate>2008-05-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "primary_doc.xml", "description": "PRIMARY DOCUMENT", "secsgml_size_bytes": 4400}]
X0202
4
2008-05-22
0
0000726517
MERCHANTS BANCSHARES INC
mbvt
0001220433
BOUYEA PETER A
275 KENNEDY DRIVE
SOUTH BURLINGTON
VT
05403
1
0
0
0
Common Stock
2008-05-22
4
P
0
1000
22.6
A
80832
D
Common Stock
1317
I
By Spouse
This Form 4 reflects the correction of an administrative error in reporting Mr. Bouyea's direct holdings of Merchants Bancshares, Inc. stock on a Form 4 filed on March 31, 2008.
This Form 4 reflects the correction of an administrative error in omitting shares owned by Mr. Bouyea's wife on Forms 4 filed by Mr. Bouyea on March 7, 2008 through May 15, 2008. As reported on Merchants Bancshares, Inc.'s proxy statement dated April 1, 2008, Mr. Bouyea disclaims beneficial ownership of these shares, as he does not control the voting or investment of the shares.
Lisa A. Razo, P.O.A. for Peter A. Bouyea
2008-05-27