SAN 6-K

FORM 6-K

SECURITIES ANDEXCHANGE COMMISSION

Washington, D.C.20549

Report of ForeignIssuer

Pursuant to Rule13a-16 or 15d-16 of

the SecuritiesExchange Act of 1934

For the month of August, 2026

Commission File Number: 001-12518

Banco Santander, S.A.

(Exact name of registrant as specified in itscharter)

Ciudad GrupoSantander

28660 Boadilladel Monte (Madrid) Spain

(Address of principalexecutive 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. (the “Bank” or “Banco Santander”), in compliance with the provisions of the Securities Market legislation, hereby announces the following:

OTHERRELEVANT INFORMATION

Further to the inside information notice dated 3 February 2026 (registry number 3071) and the other relevant information notices dated 23 April 2026 and 5 August 2026 (registry numbers 40405 and 42338), concerning the acquisition of Webster Financial Corporation (the “Acquisition”), the Bank hereby informs that the Acquisition was completed on 20 August 2026 on the terms previously announced.

Today, the capital increase through non-cash contributions approved at the Bank’s ordinary general shareholders’ meeting held on 27 March 2026 (the “Capital Increase”) has been executed on the definitive terms and conditions set out below:

Today, the public deed of execution of the Capital Increase will be filed for registration with the Commercial Registry of Santander and admission to trading of the New Shares on the Spanish Stock Exchanges will be requested. In addition, admission to trading of the New Shares will be requested on the foreign Stock Exchanges on which Banco Santander shares are listed.

Consequently, the Bank’s share capital has been set at EUR 7,509,582,970, represented by 15,019,165,940 shares with a nominal value of EUR 0.50 each. All of the shares belong to the same class and have the same rights.

Boadilla del Monte (Madrid), 20 August 2026

NOOFFER OR SOLICITATION

This communication 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 communication. By making this communication 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.