SEC Form 6-K filed by Banco Santander S.A. Sponsored ADR (Spain)

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 June, 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:

Banco Santander, S.A.

TABLE OF CONTENTS

Item 1

Banco Santander, S.A., in accordance with the provisions of the securities market legislation, communicates the following:

OTHER RELEVANT INFORMATION

BANCO SANTANDER, S.A. ANNOUNCES RESULTS OF TENDER OFFER FOR ONE SERIES OF ITS U.S. DOLLAR-DENOMINATED ADDITIONAL TIER 1 SECURITIES

Further to the communication of inside information made on May 27, 2026 (registered under number 3240), Banco Santander, S.A. (the “Offeror” or “Banco Santander”) is today announcing the results of its tender offer (the “Offer”) to purchase for cash up to $850,000,000 (the “Maximum Offer Amount”) of its outstanding 4.750% Non-Step-Up Non-Cumulative Contingent Convertible Perpetual Preferred Tier 1 Securities of Banco Santander (the “Securities”).

The Offer was made on the terms and subject to the conditions set out in the Offer to Purchase dated May 27, 2026 (the “Offer to Purchase”). Capitalized terms not otherwise defined in this announcement have the same meaning as in the Offer to Purchase.

Based on information provided by the Tender Agent, $701,600,000 aggregate principal amount of the Securities listed in the table below were validly tendered and not validly withdrawn by 5:00 p.m., New York City time, on June 9, 2026 (the “Expiration Deadline”), as more fully set forth below. Banco Santander has accepted all Securities that were validly tendered and not validly withdrawn prior to the Expiration Deadline, without proration. The Settlement Date is expected to be June 11, 2026.

Securities purchased by Banco Santander pursuant to the Offer will be cancelled and will not be re-issued or re-sold. Securities which have not been validly submitted and accepted for purchase pursuant to the Offer will remain outstanding. The aggregate principal amount of Securities outstanding after the Offer is $298,400,000.

The table below sets forth, among other things, the principal amount of the Securities validly tendered and accepted pursuant to the Offer:

D.F. King & Co., Inc. acted as tender agent for the Offer. Santander US Capital Markets LLC acted as Dealer Manager for the Offer. Questions regarding the Offer should be directed to:

Boadilla del Monte (Madrid), June 10, 2026

Disclaimer

This announcement and the Offer to Purchase (including the documents incorporated by reference therein) contain important information which should be read carefully. If you are in any doubt as to the contents of this announcement or the Offer to Purchase or the action you should take, you are recommended to seek your own financial, legal, regulatory and tax advice, including as to any tax consequences, immediately from your stockbroker, bank manager, attorney, accountant or other independent financial, tax or legal adviser.

Additional Information in Respect of the Offer and Where to Find It

Banco Santander has filed with the Securities and Exchange Commission (the “SEC”) a tender offer statement on Schedule TO, accompanied by the Offer to Purchase and related documents relating to the Offer. Holders are advised to read carefully the tender offer statement, the Offer to Purchase and other documents which Banco Santander will file with the SEC as they contain important information about the Offer. Copies of these documents are available for free by visiting EDGAR on the SEC website at www.sec.gov. In addition, copies of the Schedule TO and the documents filed with it may be obtained free of charge by contacting Investor Relations, Ciudad Grupo Santander, Avenida de Cantabria s/n, 28660 Boadilla del Monte, Madrid, Spain (telephone: (011) 34-91-259-6520).

Forward-Looking Statements

This announcement contains certain forward-looking statements within the meaning of Section 21E of the US Securities Exchange Act of 1934, as amended, and section 27A of the US Securities Act of 1933, as amended, with

respect to Banco Santander, S.A. together with its subsidiaries (the “Group”) and its current goals and expectations. Statements that are not historical or current facts, including statements about the Group’s or its directors’ and/or management’s beliefs and expectations, are forward looking statements. Words such as, without limitation, ‘believes’, ‘achieves’, ‘anticipates’, ‘estimates’, ‘expects’, ‘targets’, ‘should’, ‘intends’, ‘aims’, ‘projects’, ‘plans’, ‘potential’, ‘will’, ‘would’, ‘could’, ‘considered’, ‘likely’, ‘may’, ‘seek’, ‘estimate’, ‘probability’, ‘goal’, ‘objective’, ‘deliver’, ‘endeavour’, ‘prospects’, ‘optimistic’ and similar expressions or variations on these expressions are intended to identify forward looking statements. These statements concern or may affect future matters, including but not limited to matters, influences and factors which are beyond the Group’s control. Please refer to the latest Annual Report on Form 20-F filed by Banco Santander, S.A. with the US Securities and Exchange Commission (the SEC), which is available on the SEC’s website at www.sec.gov, for a discussion of certain factors and risks. Banco Santander, S.A. may also make or disclose written and/or oral forward-looking statements in other written materials and in oral statements made by the directors, officers or employees of Banco Santander, S.A. to third parties, including financial analysts.

Except as required by any applicable law or regulation, the forward-looking statements contained in this document are made as of today’s date, and the Group expressly disclaims any obligation or undertaking to release publicly any updates or revisions to any forward looking statements contained in this document whether as a result of new information, future events or otherwise. The information, statements and opinions contained in this document do not constitute a public offer under any applicable law or an offer to sell any securities or financial instruments or any advice or recommendation with respect to such securities or financial instruments.

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.