Report of Foreign Issuer Pursuant to Rule 13a-16 or 15d-16 (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 June, 2021

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

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):

Yes

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):

Yes

Banco Santander, S.A.

TABLE OF CONTENTS

Item

Letter to Shareholders of Banco Santander Mexico, S.A. dated May 31, 2021

Item 1

May 31, 2021

Update to the Rebuttal Letter to ISS´s recommendations of Banco Santander México, S.A. dated 26 May 2021.

Dear Santander Mexico shareholder:

We would like to follow up on the Rebuttal letter dated 26 May 2021 informing you about the publication of Glass Lewis’ report published on May 28th.

In the light of the above-mentioned report, whereby Glass Lewis recommends that shareholders vote FOR the proposal in Item 2 of OESM, which relates to the Cancelation of the Registration of shares in the National Securities Registry and Delisting from Mexican Stock Exchange, we wish to reiterate and confirm that Santander Spain believes it is in the best interest of the shareholders of Santander México to vote in favor of the above mentioned Item 2 and have therefore the possibility of the delisting offer being confirmed.

Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the Rebuttal Letter.

Very truly yours,

BANCO SANTANDER, S.A.

By:

/s/ Sergio Gámez

Name:	Sergio Gámez
Title:	Global Head of Shareholders and Investor Relations Santander

IMPORTANT INFORMATION FOR INVESTORS ABOUT THE PROPOSED TRANSACTION

The tender offer described in this communication has not yet commenced. This communication is provided for informational purposes only and does not constitute an offer to purchase or the solicitation of an offer to sell any common stock (including any American Depositary Share representing any common stock) or other securities. If and at the time a tender offer is commenced, Santander Spain (and/or one or more of its affiliates, as applicable) intends to file with the U.S. Securities and Exchange Commission (the “SEC”) a Tender Offer Statement on Schedule TO containing an offer to purchase, a form of letter of transmittal and other documents relating to the tender offer, and Santander México will file with the SEC a Solicitation/Recommendation Statement on Schedule 14D-9 with respect to the tender offer. Santander Spain intends to mail these documents to Santander México shareholders. Santander Spain will also file before the Comisión Nacional Bancaria y de Valores (“CNBV”) an informative brochure in connection with the transaction and the prospective offer as required under applicable law.

INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE TENDER OFFER STATEMENT, OFFER TO PURCHASE, SOLICITATION/ RECOMMENDATION STATEMENT AND ALL OTHER RELEVANT DOCUMENTS THAT WILL BE FILED WITH THE SEC AND THE CNBV REGARDING THE PROPOSED TRANSACTION CAREFULLY BEFORE MAKING A DECISION CONCERNING THE TENDER OFFER AS THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE TENDER OFFER.

Such documents, and other documents filed by Santander Spain and Santander México, may be obtained without charge after they have been filed at the SEC’s website at www.sec.gov and through the CNBV´s website at www.cnbv.gob.mx. The offer to purchase and related materials may also be obtained (when available) for free by contacting the information agent for the tender offer.

This communication shall not constitute a tender offer in any country or jurisdiction in which such offer would be considered unlawful or otherwise violate any applicable laws or regulations, or which would require Santander Spain or any of its affiliates to change or amend the terms or conditions of such offer in any manner, to make any additional filing with any governmental or regulatory authority or take any additional action in relation to such offer.

Cautionary Statement Regarding Forward-Looking Statements.

This communication contains “forward-looking statements” as per the meaning of the US Private Securities Litigation Reform Act of 1995. These statements may be identified by words like expect, project, anticipate, should, intend, probability, risk, target, goal, objective, estimate, future and similar expressions and include, but are not limited to, statements that are predictive in nature and depend upon or refer to future events, conditions, circumstances or the future performance of Santander Spain or Santander México or their respective affiliates, including as a result of the implementation of the transactions described herein. These statements are based on management’s current expectations and are inherently subject to uncertainties and changes in circumstance and a number of risks, uncertainties and other important factors may cause actual developments and results to differ materially from expectations. Risks and uncertainties include, among other things, risks related to the tender offer, including uncertainties as to how many of Santander México shareholders will tender their shares in the tender offer; general economic or industry conditions of areas where Santander Spain or Santander México have significant operations or investments (such as a worse economic environment; higher volatility in capital markets; inflation or deflation; changes in demographics, consumer spending, investment or saving habits; and the effects of the covid-19 pandemic on the global economy); exposure to various market risks (particularly interest rate risk, foreign exchange rate risk, equity price risk and risks associated with the replacement of benchmark indices); potential losses from early repayments on loan and investment portfolios, declines in value of collateral securing loan portfolios, and counterparty risk; political stability in Spain, the United Kingdom, other European countries, Latin America and the US; changes in legislation, regulations, taxes, including regulatory capital and liquidity requirements, especially in view of the UK exit of the European Union and increased regulation in response to financial crisis; the ability to integrate successfully acquisitions and related challenges that result from the inherent diversion of management’s focus and resources from other strategic opportunities and operational matters; and changes in access to liquidity and funding on acceptable terms, in particular if resulting from credit spread shifts or downgrades in credit ratings; and other risks and uncertainties discussed in (i) Santander México’s filings with the SEC, including the “Risk Factors” and “Special Note Regarding Forward-Looking Statements” sections of Santander México’s most recent annual report on Form 20-F and (ii) Santander Spain’s filings with the SEC, including the “Risk Factors” and “Cautionary Statement Regarding Forward-Looking Statements” sections of Santander Spain’s most recent annual report on Form 20-F. You can obtain copies of Santander Spain’s and Santander México’s filings with the SEC for free at the SEC’s website (www.sec.gov). Other factors that may cause actual results to differ materially include those that will be set forth in the Tender Offer Statement on Schedule TO, the Solicitation/Recommendation Statement on Schedule 14D-9 and other tender offer documents filed by Santander Spain and Santander México. All forward-looking statements in this communication are qualified in their entirety by this cautionary statement.

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.