FORM 6-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16 UNDER

THE SECURITIES EXCHANGE ACT OF 1934

For the month of April, 2018

Commission File Number: 001-12518

BANCO SANTANDER, S.A.

Ciudad Grupo Santander

28660 Boadilla del Monte

Madrid

Spain

(Address of principal executive offices)

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  ☒                    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):  ☐

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):  ☐

EXPLANATORY NOTE

In connection with the issuance by Banco Santander, S.A. of $1,250,000,000 aggregate principal amount of 3.848% Senior Non Preferred Fixed Rate Notes due 2023, $1,250,000,000 aggregate principal amount of 4.379% Senior Non Preferred Fixed Rate Notes due 2028 and $500,000,000 aggregate principal amount of Senior Non Preferred Floating Rate Notes due 2023, Banco Santander, S.A. is filing the following documents solely for incorporation into the Registration Statement on Form F-3 (File Nos. 333-217116):

Exhibit List

SIGNATURES

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.