6-K

FORM 6-K

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Report of Foreign Issuer

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

the Securities Exchange Act of 1934

For the month of May, 2026

Commission File Number: 001-12518

Banco Santander, S.A.

(Exact name of registrant as specified in itscharter)

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:

Banco Santander, S.A.

TABLE OF CONTENTS

Item1

Banco Santander, S.A. (“Banco Santander” or the “Bank”), in compliance with the Securities Market legislation, hereby announces the following:

OTHERRELEVANT INFORMATION

Further to the other relevant information notice dated 23 April 2026 (official registry number 40404) regarding the temporary suspension of the share buyback programme (the “Buyback Programme”), Banco Santander hereby announces that the resumption of the Buyback Programme will occur on 28 May 2026. Consequently, the indicative duration of the Buyback Programme is now expected to run to 21 August, inclusive.

Boadilla del Monte (Madrid), 27 May 2026

IMPORTANT INFORMATION

No offer or solicitation

This document does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended (the “Securities Act”). No investment activity should be undertaken on the basis of the information contained in this document. By making this document available, no advice or recommendation is being given to buy, sell or otherwise deal in any securities or investments whatsoever.

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.