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SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

Report of Foreign Issuer

Pursuant to Rule 13a-16 or 15d-16

of the Securities Exchange Act of 1934

For the month of September, 2014

Commission File Number: 001-12518

Banco Santander, S.A.

(Exact name of registrant as specified in its charter)

Ciudad Grupo Santander

28660 Boadilla del Monte (Madrid) Spain

(Address of principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

Form 20-F  x            Form 40-F  ¨

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):

Yes  ¨            No   x

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):

Yes  ¨            No   x

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Banco Santander, S.A.

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MATERIAL FACT

Banco Santander, S.A. (the “Bank”) hereby announces that as a result of the change in Standard & Poor’s Ratings Services general criterion with respect to issuances of preferred stock and other capital instruments, the latter published on 30 September 2014 an updating of its ratings.

Following the change in criterion, the revised ratings for the Bank are detailed below:

Boadilla del Monte (Madrid), 30 September 2014

Banco Santander, S.A.—Domicilio Social: Paseo de Pereda, 9-12. 39004 SANTANDER—R. M. de Santander, Hoja 286, Folio 64, Libro 5º de Sociedades, Inscripción 1ª. C.I.F. A-39000013

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.