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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-10-30</periodOfReport>

    <issuer>
        <issuerCik>0001618921</issuerCik>
        <issuerName>Walgreens Boots Alliance, Inc.</issuerName>
        <issuerTradingSymbol>WBA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001179715</rptOwnerCik>
            <rptOwnerName>SKINNER JAMES A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALGREENS BOOTS ALLIANCE, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>108 WILMOT ROAD</rptOwnerStreet2>
            <rptOwnerCity>DEERFIELD</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60015</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Exec. Chairman of the Board</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-10-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>66863</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>68269</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-11-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>373.957</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>84.68</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>373.957</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>75357.6</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On October 30, 2015, the Compensation Committee determined that the performance criteria applicable to this restricted stock unit award granted on January 15, 2015 (not a derivative security within the meaning of Rule 16a-1(c) upon grant) under the Walgreens Boots Alliance, Inc. 2013 Omnibus Incentive Plan, as amended (together with the related award agreement and the applicable election forms thereunder, the &quot;Plan&quot;) had been satisfied. The shares underlying these restricted stock units will vest on January 15, 2018, subject to the terms and conditions of the Plan.</footnote>
        <footnote id="F2">Includes shares underlying restricted stock units issued in lieu of dividends (through September 30, 2015) on outstanding restricted stock units, including 816 shares with respect to the restricted stock unit award described in note (1) above.</footnote>
        <footnote id="F3">The phantom stock was issued as non-employee director compensation under the  Plan for service as a non-employee director from November 1, 2014 through January 8, 2015, and each unit of phantom stock is the economic equivalent of one share of the company's common stock.</footnote>
        <footnote id="F4">To be settled in two installments, the first of which occurs within thirty days following termination of service as a director, and the second, one year after the first settlement date (subject to the terms and conditions of the Plan as in effect from time to time).</footnote>
        <footnote id="F5">Includes phantom stock units issued in lieu of dividends (through September 30, 2015) on outstanding phantom stock units.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Kelsey Chin, Attorney-in-Fact</signatureName>
        <signatureDate>2015-11-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2015-10-30
0001618921
Walgreens Boots Alliance, Inc.
WBA
0001179715
SKINNER JAMES A
C/O WALGREENS BOOTS ALLIANCE, INC.
108 WILMOT ROAD
DEERFIELD
IL
60015
1
1
0
0
Exec. Chairman of the Board
Common Stock
2015-10-30
4
A
0
66863
0
A
68269
D
Phantom Stock Units
2015-11-01
4
A
0
373.957
84.68
A
Common Stock
373.957
75357.6
D
On October 30, 2015, the Compensation Committee determined that the performance criteria applicable to this restricted stock unit award granted on January 15, 2015 (not a derivative security within the meaning of Rule 16a-1(c) upon grant) under the Walgreens Boots Alliance, Inc. 2013 Omnibus Incentive Plan, as amended (together with the related award agreement and the applicable election forms thereunder, the "Plan") had been satisfied. The shares underlying these restricted stock units will vest on January 15, 2018, subject to the terms and conditions of the Plan.
Includes shares underlying restricted stock units issued in lieu of dividends (through September 30, 2015) on outstanding restricted stock units, including 816 shares with respect to the restricted stock unit award described in note (1) above.
The phantom stock was issued as non-employee director compensation under the  Plan for service as a non-employee director from November 1, 2014 through January 8, 2015, and each unit of phantom stock is the economic equivalent of one share of the company's common stock.
To be settled in two installments, the first of which occurs within thirty days following termination of service as a director, and the second, one year after the first settlement date (subject to the terms and conditions of the Plan as in effect from time to time).
Includes phantom stock units issued in lieu of dividends (through September 30, 2015) on outstanding phantom stock units.
/s/ Kelsey Chin, Attorney-in-Fact
2015-11-03