<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-07-08</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000764478</issuerCik>
        <issuerName>BEST BUY CO INC</issuerName>
        <issuerTradingSymbol>BBY</issuerTradingSymbol>
</issuer>
<reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001191193</rptOwnerCik>
            <rptOwnerName>LINTON MIACHEL A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>EVP - CMO</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeSecurity><securityTitle><value>Common Stock</value>
</securityTitle>
<transactionDate><value>2003-07-08</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>M</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>35000</value>
</transactionShares>
<transactionValue><value>21.17</value>
</transactionValue>
<transactionAcquiredDisposedCode><value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>36500</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</nonDerivativeSecurity>
<nonDerivativeSecurity><securityTitle><value>Common Stock</value>
</securityTitle>
<transactionDate><value>2003-07-08</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>S</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>10000</value>
</transactionShares>
<transactionValue><value>45.20</value>
</transactionValue>
<transactionAcquiredDisposedCode><value>D</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>26500</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</nonDerivativeSecurity>
<nonDerivativeSecurity><securityTitle><value>Common Stock</value>
</securityTitle>
<transactionDate><value>2003-07-08</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>S</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>18000</value>
</transactionShares>
<transactionValue><value>46.00</value>
</transactionValue>
<transactionAcquiredDisposedCode><value>D</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>8500</value>
<footnoteId id="F1"></footnoteId>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</nonDerivativeSecurity>
<derivativeSecurity><securityTitle><value>Stock Option (Right to Buy)</value>
</securityTitle>
<conversionOrExercisePrice><value>21.17</value>
</conversionOrExercisePrice>
<transactionDate><value>2003-07-08</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>M</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>35000</value>
</transactionShares>
<transactionValue><value>0</value>
</transactionValue>
<transactionAcquiredDisposedCode><value>D</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<exerciseDate><value>1999-01-04</value>
</exerciseDate>
<expirationDate><value>2009-01-03</value>
</expirationDate>
<underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares><value>35000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>0</value>
<footnoteId id="F2"></footnoteId>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</derivativeSecurity>
<footnotes><footnote id="F1">The reporting person also indirectly beneficially owns 695 shares (as of 6/30/03) of Best Buy common stock held in a 401(k) retirement savings account.</footnote>
<footnote id="F2">The reporting person also directly beneficially owns the following employee stock options to purchase Best Buy common stock (options vest in four equal annual installments beginning one year from the date of grant): 11,250 options at $34.45, granted 2/28/00, expire 2/27/10; 33,750 options at $46.75, granted 4/14/00, expire 4/13/10; 33,750 options at $37.06, granted 4/27/01, expire 4/26/11; 32,625 options at $51.27, granted 4/11/02, expire 4/10/12; and 20,000 options at $28.67, granted 1/16/03, expire 1/15/13.</footnote>
</footnotes>
<ownerSignature><signatureName>/s/ Nancy J. Wigchers                            Attorney-in-fact for Michael A. Linton</signatureName>
<signatureDate>2003-07-09</signatureDate>
</ownerSignature>
</ownershipDocument>

