SAN 6-K

FORM 6-K

SECURITIES AND EXCHANGE 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 March, 2025

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 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

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

OTHER RELEVANT INFORMATION

Reference is made to our notice of inside information of 5 February 2025 (official registry number 2584) (the “Buy-back Commencement Communication”), relating to the buyback programme of own shares (the “Buy-back Programme”) approved by the Board of Directors of Banco Santander.

Pursuant to article 5 of Regulation (EU) no. 596/2014 on Market Abuse of 16 April 2014, and articles 2.2 and 2.3 of Commission Delegated Regulation (EU) 2016/1052, of 8 March 2016, the Bank informs of the transactions carried out over its own shares between 20 and 26 March 2025 (both inclusive).

The cash amount of the shares purchased to 26 March 2025 as a result of the execution of the Buy-back Programme amounts to 573,513,880 Euros, which represents approximately 36.1% of the maximum investment amount of the Buy-back Programme. The programme was announced together with its other characteristics through the Buy-back Commencement Communication. With these purchases, the Bank has repurchased approximately 13.2% of its outstanding shares as of 2021.

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Issuer name: Banco Santander, S.A. - LEI 5493006QMFDDMYWIAM13

Reference of the financial instrument: ordinary shares - Code ISIN ES0113900J37

Detailed information of the transactions carried out within the referred period is attached as Annex I.

Boadilla del Monte (Madrid), 27 March 2025

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ANNEX I

Detailed information on each of the transactions carried out within the context of the Buy-back Programme between 20/03/2025 and 26/03/2025 (both inclusive)

(https://www.santander.com/content/dam/santander-com/es/documentos/cumplimiento/do-anexo-i-20-mar-a-26-mar-2025.pdf)

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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.