Form 6-K Banco Santander, S.A. For: Apr 15

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

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, 2026.

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 ☒   Form 40-F ☐

Banco Santander, S.A.

EXPLANATORY NOTE

In connection with the issuance by Banco Santander, S.A. of $400,000,000 aggregate principal amount of Senior Non Preferred Floating Rate Notes due 2029, $1,000,000,000 aggregate principal amount of 4.600% Senior Non Preferred Fixed Rate Notes due 2029, $1,250,000,000 aggregate principal amount of 4.867% Senior Non Preferred Fixed Rate Notes due 2031, and $1,000,000,000 aggregate principal amount of 5.437% Senior Non Preferred Fixed Rate Notes due 2036, Banco Santander, S.A. is filing the following documents solely for incorporation into the Registration Statement on Form F-3 (File No. 333-293987):

Exhibit List

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.

Date: April 15, 2026