424B5

424B5 1 d137445d424b5.htm 424B5 424B5

Filed Pursuant to Rule 424(b)(5)
Registration Statement No. 333-293987

The information in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities nor do they seek an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion

Preliminary Prospectus Supplement dated April 8, 2026

PRELIMINARY PROSPECTUS SUPPLEMENT

(to prospectus dated March 4, 2026)

LOGO

Banco Santander, S.A.

$ Senior Non Preferred Floating Rate Notes due 2029

$ % Senior Non Preferred Fixed Rate Notes due 2029

$ % Senior Non Preferred Fixed Rate Notes due 2031

$ % Senior Non Preferred Fixed Rate Notes due 2036

We are offering $ principal amount of Senior Non Preferred Floating Rate Notes due 2029 (the “2029 Floating Rate Notes”), $ principal amount of % Senior Non Preferred Fixed Rate Notes due 2029 (the “2029 Fixed Rate Notes”), $ principal amount of % Senior Non Preferred Fixed Rate Notes due 2031 (the “2031 Fixed Rate Notes”), and $ principal amount of % Senior Non Preferred Fixed Rate Notes due 2036 (the “2036 Fixed Rate Notes” and, together with the 2029 Floating Rate Notes, the 2029 Fixed Rate Notes and the 2031 Fixed Rate Notes, the “Notes”).

The 2029 Floating Rate Notes will bear interest at the 2029 Floating Rate Interest Rate (as defined herein). From and including the date of issuance, interest will be payable quarterly in arrears on the 2029 Floating Rate Notes on January , April , July and October of each year, beginning on July , 2026. Unless we redeem the 2029 Floating Rate Notes earlier, the 2029 Floating Rate Notes will be due on April , 2029.

The 2029 Fixed Rate Notes will bear interest at a rate of % per year. From and including the date of issuance, interest will be payable semi-annually in arrears on the 2029 Fixed Rate Notes on April and October of each year, beginning on October , 2026. Unless we redeem the 2029 Fixed Rate Notes earlier, the 2029 Fixed Rate Notes will be due on April , 2029.

The 2031 Fixed Rate Notes will bear interest at a rate of % per year. From and including the date of issuance, interest will be payable semi-annually in arrears on the 2031 Fixed Rate Notes on April and October of each year, beginning on October , 2026. Unless we redeem the 2031 Fixed Rate Notes earlier, the 2031 Fixed Rate Notes will be due on April , 2031.

The 2036 Fixed Rate Notes will bear interest at a rate of % per year. From and including the date of issuance, interest will be payable semi-annually in arrears on the 2036 Fixed Rate Notes on April and October of each year, beginning on October , 2026. Unless we redeem the 2036 Fixed Rate Notes earlier, the 2036 Fixed Rate Notes will be due on April , 2036.

The Notes will be issued in minimum denominations of $200,000 and integral multiples of $200,000 in excess thereof.

The payment obligations of Banco Santander, S.A. (“Banco Santander”) in respect of principal under the Notes constitute direct, unconditional, unsubordinated and unsecured senior non preferred obligations (créditos ordinarios no preferentes) of Banco Santander and, in accordance with Additional Provision 14.2 of Law 11/2015 (as defined herein) (but subject to any other ranking that may apply as a result of any mandatory provision of law (or otherwise)), upon the insolvency of Banco Santander, such payment obligations rank (i) pari passu among themselves and with any other Senior Non Preferred Liabilities (as defined herein), (ii) junior to the Senior Higher Priority Liabilities (as defined herein) (and, accordingly, upon the insolvency of Banco Santander,

the payment obligations of Banco Santander in respect of principal under the Notes will be met after payment in full of the Senior Higher Priority Liabilities) and (iii) senior to any present and future subordinated obligations (créditos subordinados) of Banco Santander in accordance with Article 281 of the Spanish Insolvency Law (as defined herein).

By its acquisition of the Notes, each holder (which, for the purposes of this clause, includes each holder of a beneficial interest in the Notes) acknowledges, accepts, consents to and agrees to be bound by the terms of the Notes related to the exercise of the Bail-in Power (as defined herein) set forth under “Description of Debt Securities—Agreement and Acknowledgement with Respect to the Exercise of the Bail-in Power” in the accompanying prospectus. See “Notice to Investors” on page S-i of this prospectus supplement for further information.

The Notes are not deposits or savings accounts and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency of the Kingdom of Spain, the United States or any other jurisdiction.

Subject to the terms and conditions under “Description of the Notes—Redemption and Repurchase,” we may redeem the Notes of any series, in whole but not in part, at 100% of their principal amount plus accrued and unpaid interest (if any), (i) at any time upon the occurrence of a Tax Event (as defined herein), (ii) at any time upon the occurrence of a TLAC/MREL Disqualification Event (as defined herein) or (iii) if 75% or more of the initial aggregate principal amount of such series of Notes have been redeemed or purchased by, or on behalf of, Banco Santander and cancelled. We may not redeem the Notes of any series under other circumstances, and there are no put rights with respect to the Notes of any series. See “Description of the Notes—Redemption and Repurchase.”

We intend to apply to list the Notes on the New York Stock Exchange in accordance with its rules.

Investing in the Notes involves risks. See “Risk Factors” beginning on page S-16 of this prospectus supplement, page 5 of the accompanying prospectus as well as those discussed under the heading “Risk Factors” in the Group’s Annual Report on Form 20-F for the year ended December 31, 2025, which is incorporated by reference herein.

The Notes are not intended to be offered, sold or otherwise made available and should not be offered, sold or otherwise made available to retail clients (as defined in Directive 2014/65/EU of the European Parliament and of the Council on Markets in Financial Instruments (“MiFID II”) and Regulation (EU) No. 2017/565 as it forms part of the domestic law of the United Kingdom by virtue of the European Union (Withdrawal) Act of 2018 (“EUWA”)) in the European Economic Area or in the United Kingdom. Prospective investors are referred to the section headed “Important Information” on page S-v of this prospectus supplement.

Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

Price to Public

Underwriting
Discount

Proceeds to us
(before expenses)

Per 2029 Floating Rate Note

Total 2029 Floating Rate Notes

Per 2029 Fixed Rate Note

Total 2029 Fixed Rate Notes

Per 2031 Fixed Rate Note

Total 2031 Fixed Rate Notes

Per 2036 Fixed Rate Note

Total 2036 Fixed Rate Notes

Total

The initial public offering prices set forth above do not include accrued interest, if any. Interest on the Notes will accrue from the expected date of issuance, which is April , 2026. See “Underwriting (Conflicts of Interest).”

We expect that the Notes will be ready for delivery through the book-entry facilities of The Depository Trust Company (“DTC”) and its direct and indirect participants, including Clearstream Banking, société anonyme (“Clearstream Luxembourg”) and Euroclear Bank SA/NV (“Euroclear”) on or about April , 2026, which will be the New York business day following the pricing of the Notes (such settlement period being referred to as “T+ ”). Beneficial interests in the Notes will be shown on, and transfers thereof will be effected only through, records maintained by DTC and its participants.

Joint Bookrunners


BNP PARIBAS		Citigroup		Goldman Sachs		HSBC		J.P. Morgan


NatWest		RBC		Santander		Scotiabank		TD Securities

Prospectus Supplement dated April , 2026

TABLE OF CONTENTS

Prospectus Supplement


	Page
Notice to Investors		S-i
About this Prospectus Supplement		S-iv
Incorporation of Information by Reference		S-iv
Forward-Looking Statements		S-v
Important Information		S-v
Summary		S-1
Risk Factors		S-16
Use of Proceeds		S-20
Secured Overnight Financing Rate		S-21
Capitalization of the Group		S-22
Description of the Notes		S-23
Taxation		S-32
Underwriting (Conflicts of Interest)		S-39
Legal Opinions		S-47
Experts		S-47
Enforcement of Civil Liabilities		S-47

Prospectus


About this Prospectus			1
Use of Proceeds			2
Banco Santander, S.A.			2
Summary of Risk Factors			3
Risk Factors			5
Description of Debt Securities			43
Description of Contingent Convertible Capital Securities			71
Description of Certain Provisions Relating to Debt Securities and Contingent Convertible Capital Securities			120
Description of Ordinary Shares			126
Description of American Depositary Shares			127
Taxation			133
Benefit Plan Investor Considerations			163
Plan of Distribution (Conflicts of Interest)			165
Legal Opinions			167
Experts			167
Enforcement of Civil Liabilities			167
Where You Can Find More Information			167
Incorporation of Documents by Reference			169
Cautionary Statement on Forward-Looking Statements			170

NOTICE TO INVESTORS

Agreements and Acknowledgments of Investors, including Holders and Beneficial Owners

Notwithstanding any other term of the Notes or any other agreements, arrangements, or understandings between Banco Santander and any holder of the Notes, by its acquisition of the Notes, each holder (which includes each holder of a beneficial interest in the Notes) acknowledges, accepts, consents to and agrees:

(i) to be bound by the effect of the exercise of the Bail-in Power by the Relevant Resolution Authority (as defined herein), which may include and result in any of the following, or some combination thereof:

•

the reduction of all, or a portion, of the Amounts Due (as defined herein) on a permanent basis;

•

the conversion of all, or a portion, of the Amounts Due into Common Equity Tier 1 Instruments (as defined herein), other securities or other obligations of Banco Santander or another person (and the issue to the holder of such Common Equity Tier 1 Instruments, securities or obligations), including by means of an amendment, modification or variation of the terms of the Notes, in which case the holder agrees to accept in lieu of its rights under the Notes, any such Common Equity Tier 1 Instruments, other securities or other obligations of Banco Santander or another person;

•

the cancellation of the Notes or Amounts Due;

•

the amendment or alteration of the maturity of the Notes or amendment of the interest payable on the Notes, or the date on which the interest becomes payable, including by suspending payment for a temporary period; and

(ii) that the terms of the Notes are subject to, and may be varied, if necessary, to give effect to, the exercise of the Bail-in Power by the Relevant Resolution Authority.

“Amounts Due” means the principal amount of, premium, if any, together with any accrued but unpaid interest, and Additional Amounts (as defined herein), if any, due on the Notes. References to such amounts will include amounts that have become due and payable, but which have not been paid, prior to the exercise of the Bail-in Power by the Relevant Resolution Authority.

“Applicable Banking Regulations” means at any time the laws, regulations, requirements, guidelines and policies relating to capital adequacy, resolution and/or solvency including, among others, those giving effect to the MREL and the TLAC or any equivalent or successor principles, then applicable to Banco Santander and/or the Group including, without limitation to the generality of the foregoing, the CRD, the BRRD, the SRM Regulation and those regulations, requirements, guidelines and policies relating to capital adequacy, resolution and/or solvency of the Regulator then applicable to Banco Santander and/or the Group including, among others, those giving effect to the MREL and the TLAC or any equivalent or successor principles, in each case to the extent then in effect in the Kingdom of Spain (whether or not such regulations, requirements, guidelines or policies have the force of law and whether or not they are applied generally or specifically to Banco Santander and/or the Group).

“Bail-in Power” means any power existing from time to time under, and exercised in compliance with, any laws, regulations, rules or requirements in effect in the Kingdom of Spain, relating to (i) the transposition of the BRRD (as defined herein), including its article 48 and, when applicable, its article 59 (therefore, the write-down capital conversion power at the point of non-viability) (including but not limited to, Law 11/2015, RD 1012/2015 (as defined herein) and any other implementing regulations),

S-i

(ii) the SRM Regulation (as defined herein) and (iii) the instruments, rules or standards created thereunder, pursuant to which any obligation of a Regulated Entity (as defined herein) (or an affiliate of such Regulated Entity) can be reduced, cancelled, suspended, modified, or converted into shares, other securities, or other obligations of such Regulated Entity (or affiliate of such Regulated Entity).

“BRRD” means Directive 2014/59/EU of 15 May 2014 establishing a framework for the recovery and resolution of credit institutions and investment firms, as amended or replaced from time to time.

“CET1 Capital” means at any time, the common equity tier 1 capital of Banco Santander or the Group, respectively, as calculated in accordance with Chapter 2 (Common Equity Tier 1 Capital) of Title I (Elements of own funds) of Part Two (Own Funds and Eligible Liabilities) of the CRR and/or Applicable Banking Regulations at such time, including any applicable transitional, phasing in or similar provisions.

“Common Equity Tier 1 Instruments” means instruments qualifying as CET1 Capital.

“CRD” means any or any combination of the CRD Directive, the CRR and any CRD Implementing Measures.

“CRD Directive” means Directive 2013/36/EU of the European Parliament and of the Council of 26 June 2013 on access to the activity of credit institutions and the prudential supervision of credit institutions, as amended or replaced from time to time.

“CRD Implementing Measures” means any regulatory capital rules implementing the CRD Directive or the CRR which may from time to time be introduced, including, but not limited to, delegated or implementing acts (regulatory technical standards) adopted by the European Commission, national laws and regulations, and regulations and guidelines issued by the Regulator, the European Banking Authority or any other relevant authority, which are applicable to Banco Santander (on a standalone basis) or the Group (on a consolidated basis) and which prescribe the requirements to be fulfilled by financial instruments for inclusion in the regulatory capital or the minimum requirement for own funds and eligible liabilities, as the case may be, of Banco Santander (on a standalone basis) or the Group (on a consolidated basis).

“CRR” means Regulation (EU) No. 575/2013 of the European Parliament and of the Council of 26 June 2013 on the prudential requirements for credit institutions, as amended or replaced from time to time.

“Law 11/2015” means Law 11/2015 of 18 June, on recovery and resolution of credit institutions and investment firms (Ley 11/2015, de 18 de junio, de recuperación y resolución de entidades de crédito y empresas de servicios de inversión), as amended or replaced from time to time.

“MREL” means the “minimum requirement for own funds and eligible liabilities” for credit institutions under the BRRD, set in accordance with Article 45 of the BRRD (as transposed in the Kingdom of Spain), Commission Delegated Regulation (EU) No. 2016/1450 of 23 May 2016, supplementing Directive 2014/59/EU of the European Parliament and of the Council with regard to regulatory technical standards specifying the criteria relating to the methodology for setting the minimum requirement for own funds and eligible liabilities and any other Applicable Banking Regulations.

“RD 1012/2015” means Royal Decree 1012/2015, of 6 November, developing Law 11/2015, of 18 June, on the recovery and resolution of credit institutions and investment firms (Real Decreto 1012/2015, de 6 de noviembre, por el que se desarrolla la Ley 11/2015, de 18 de junio, de recuperación y resolución de entidades de crédito y empresas de servicios de inversión), as amended or replaced from time to time.

S-ii

“Regulated Entity” means any entity to which BRRD, as implemented in the Kingdom of Spain (including but not limited to, Law 11/2015, RD 1012/2015 and any other implementing regulations), or any other Spanish law relating to the Bail-in Power, applies, which includes, certain credit institutions, investment firms, and certain of their parent or holding companies.

“Regulator” means the European Central Bank, the Bank of Spain, the Relevant Resolution Authority or such other or successor authority exercising primary bank supervisory authority or the role of primary bank resolution authority, in each case with respect to prudential or resolution matters in relation to Banco Santander and/or the Group.

“Relevant Resolution Authority” means the Spanish Fund for the Orderly Restructuring of Banks, the Bank of Spain, the European Single Resolution Board, as the case may be, according to Law 11/2015, and any other entity with the authority to exercise the Bail-in Power or any other resolution power from time to time.

“SRM Regulation” means Regulation (EU) No. 806/2014 of the European Parliament and of the Council of 15 July 2014, establishing uniform rules and a uniform procedure for the resolution of credit institutions and certain investment firms in the framework of a Single Resolution Mechanism and a Single Resolution Fund, as amended or replaced from time to time.

“TLAC” means the “total loss-absorbing capacity” requirement for global systemically important institutions under the CRR, set in accordance with Article 92a of the CRR and/or any other Applicable Banking Regulations.

By its acquisition of the Notes, each holder of the Notes (which, for the purposes of this clause, includes each holder of a beneficial interest in the Notes), to the extent permitted by the Trust Indenture Act of 1939, as amended (“Trust Indenture Act”), will waive any and all claims, in law and/or in equity, against the Trustee (as defined herein) for, agree not to initiate a suit against the Trustee in respect of, and agree that the Trustee will not be liable for, any action that the Trustee takes, or abstains from taking, in either case in accordance with the exercise of the Bail-in Power by the Relevant Resolution Authority with respect to the Notes.

By purchasing the Notes, each holder (including each beneficial owner) of the Notes shall be deemed to have authorized, directed and requested DTC and any direct participant in DTC or other intermediary through which it holds the Notes to take any and all necessary action, if required, to implement the exercise of the Bail-in Power with respect to the Notes as it may be imposed, without any further action or direction on the part of such holder.

Neither Banco Santander nor the Underwriters (as defined in “Underwriting (Conflicts of Interest)” in this prospectus supplement) have authorized anyone to provide you with information other than information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus (including any free writing prospectus issued or authorized by us). The distribution of this prospectus supplement and the accompanying prospectus and the offering of the Notes in some jurisdictions may be restricted by law. If you possess this prospectus supplement and the accompanying prospectus, you should find out about and observe these restrictions. This prospectus supplement and the accompanying prospectus are not an offer to sell the Notes and neither Banco Santander nor the Underwriters are soliciting an offer to buy the Notes in any jurisdiction where the offer or sale is not permitted or where the person making the offer or sale is not qualified to do so or from any person to whom it is not permitted to make such offer or sale. We refer you to the information under “Underwriting (Conflicts of Interest)” in this prospectus supplement. You should assume that the information contained in this prospectus supplement, the accompanying prospectus and the information incorporated by reference into this prospectus supplement is accurate only as of their respective dates.

S-iii

ABOUT THIS PROSPECTUS SUPPLEMENT

In this prospectus supplement, we use the following terms:

•

“we,” “our” and “us” or “Banco Santander” means Banco Santander, S.A.;

•

“Group” means Banco Santander, S.A. and its consolidated subsidiaries;

•

“dollars” and “$” refer to the currency of the United States;

•

“euro” and “€” refer to the currency of the Member States of the European Union (“EU”) that have adopted the single currency in accordance with the treaty establishing the European Community, as amended; and

•

“SEC” refers to the U.S. Securities and Exchange Commission.

This document is not a prospectus for the purposes of (i) Regulation (EU) No. 2017/1129 of the European Parliament and of the Council of 14 June 2017 on the prospectus to be published when securities are offered to the public or admitted to trading on a regulated market (the “Prospectus Regulation”) or (ii) the United Kingdom Public Offers and Admissions to Trading Regulations 2024 (“POATR”) and Prospectus Rules: Admission to Trading on a Regulated Market sourcebook (“PRM”). This document has been prepared on the basis that all offers of the Notes offered hereby made to persons in the European Economic Area (“EEA”) or in the United Kingdom (“UK”) will be made pursuant to an exemption from or in compliance with the relevant requirement to produce a prospectus in connection with offers of such Notes.

INCORPORATION OF INFORMATION BY REFERENCE

This prospectus supplement is part of a registration statement on Form F-3 (File No. 333-293987) filed with the SEC under the Securities Act of 1933, as amended (the “Securities Act”). This prospectus supplement omits some information contained in the registration statement in accordance with SEC rules and regulations. You should review the information in and exhibits to the registration statement for further information on us and the Notes. Statements in this prospectus supplement concerning any document we filed or will file as an exhibit to the registration statement or that we otherwise filed with the SEC are not intended to be comprehensive and are qualified in their entirety by reference to these filings. You should review the complete document to evaluate these statements.

The SEC allows us to “incorporate by reference” the information that we file with the SEC. This permits us to disclose important information to you by referring to these filed documents. Any information referred to in this way is considered part of this prospectus supplement and accompanying prospectus, and any information that we file with the SEC after the date of this prospectus supplement will automatically be deemed to update and supersede this information.

We incorporate by reference (i) the Group’s Report on Form 6-K filed with the SEC on April 1, 2026 relating to certain recast financial information as a result of certain changes to the presentation of the Group’s financial information (accession no. 0000891478-26-000037) (the “Recast 6-K”), (ii) the Group’s 2025 Annual Report on Form 20-F for the year ended December 31, 2025 filed with the SEC on February 27, 2026 (except with respect to the information and the audited financial statements included therein and superseded by the information and the audited financial statements included in the Recast 6-K) and (iii) Sections 3 and 4 under “Part 1. Consolidated Directors’ Report-Economic and Financial Review” in the Group’s annual report on Form 20-F for the year ended December 31, 2024 filed with the SEC on February 28, 2025 (except with respect to the information included therein and superseded by the information included in the Recast 6-K).

We also incorporate by reference all subsequent annual reports of the Group filed on Form 20-F and any future filings made with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of

S-iv

1934, as amended, and certain reports on Form 6-K, if they state that they are incorporated by reference into the registration statement of which this prospectus supplement forms a part, that we furnish to the SEC after the date of this prospectus supplement and until the Underwriters sell all of the Notes.

Upon written or oral request, we will provide free of charge a copy of any or all of the documents that we incorporate by reference into this prospectus supplement, other than exhibits which are not specifically incorporated by reference into this prospectus supplement. To obtain copies you should contact us at 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

From time to time, we may make statements, both written and oral, regarding assumptions, projections, expectations, intentions or beliefs about future events. These statements constitute “forward-looking statements” for purposes of the Private Securities Litigation Reform Act of 1995. We caution that these statements may and often do vary materially from actual results. Accordingly, we cannot assure you that actual results will not differ materially from those expressed or implied by the forward-looking statements. You should read the sections entitled “Risk Factors” in this prospectus supplement and the accompanying prospectus, and “Risk Factors” and “Cautionary Statement Regarding Forward-Looking Statements” in our Annual Report on Form 20-F for the year ended December 31, 2025, which is incorporated by reference herein.

We do not undertake any obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise. In light of these risks, uncertainties and assumptions, forward-looking events discussed in this prospectus supplement and the accompanying prospectus or any information incorporated by reference, might not occur.

IMPORTANT INFORMATION

EU PRIIPs Regulation/Prohibition of Sales to EEA Retail Investors – The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the EEA. For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client, as defined in point (11) of Article 4(1) of MiFID II; (ii) a customer within the meaning of Directive (EU) 2016/97 (as amended, the “IDD”), where that customer would not qualify as a professional client as defined in point (10) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in the Prospectus Regulation. Consequently, no key information document required by Regulation (EU) No. 1286/2014 (the “EU PRIIPs Regulation”) for offering or selling the Notes or otherwise making them available to retail investors in the EEA has been prepared and therefore offering or selling the Notes or otherwise making them available to any retail investor in the EEA may be unlawful under the EU PRIIPs Regulation.

UK PRIIPs Regulation/Prohibition of Sales to UK Retail Investors – The Notes are not intended to be offered, sold, distributed or otherwise made available to and should not be offered, sold, distributed or otherwise made available to any retail investor in the United Kingdom. For these purposes, a retail investor means a person who is either one (or both) of the following: (i) not a professional client, as defined in point (8) of Article 2(1) of Regulation (EU) No 600/2014 as it forms part of domestic law by virtue of the European Union (Withdrawal Act 2018 (“EUWA”)); or (ii) not a qualified investor as defined in paragraph 15 of Schedule 1 to the Public Offers and Admissions to Trading Regulations 2024 (the “POATRs”). Consequently, no disclosure document required by the FCA Product Disclosure Sourcebook (“DISC”) for offering, selling or distributing the Notes or otherwise making them available to retail investors in the United Kingdom has been prepared and therefore offering, selling or distributing the Notes or otherwise making them available to any retail investor in the United Kingdom may be unlawful under DISC and the Consumer Composite Investments (Designated Activities) Regulations 2024.

S-v

Each Underwriter (as defined herein) (a) has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the FSMA) received by it in connection with the issue or sale of the Notes in circumstances in which Section 21(1) of the FSMA does not or would not, if it was not an authorised person, apply to Banco Santander; and (b) has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the Notes in, from or otherwise involving the United Kingdom.

MiFID II product governance/Professional investors and ECPs only target market – Solely for the purposes of each manufacturer’s product approval process, the target market assessment in respect of the Notes has led to the conclusion that: (i) the target market for the Notes is eligible counterparties and professional clients only, each as defined in MiFID II; and (ii) all channels for distribution of the Notes to eligible counterparties and professional clients are appropriate. The target market assessment indicates that the Notes are incompatible with the needs, characteristics and objectives of clients which are retail clients (as defined in MiFID II). Any person subsequently offering, selling or recommending the Notes (a “distributor”) should take into consideration the manufacturers’ target market assessment; however, a distributor subject to MiFID II is responsible for undertaking its own target market assessment in respect of the Notes (by either adopting or refining the manufacturers’ target market assessment) and determining appropriate distribution channels.

UK MiFIR product governance/Professional investors and ECPs only target market – Solely for the purposes of each manufacturer’s product approval process, the target market assessment in respect of the Notes has led to the conclusion that: (i) the target market for the Notes is eligible counterparties, as defined in the FCA Handbook Conduct of Business Sourcebook, and professional clients only, as defined in Regulation (EU) No. 600/2014 as it forms part of UK domestic law by virtue of the EUWA; and (ii) all channels for distribution of the Notes to eligible counterparties and professional clients are appropriate. The target market assessment indicates that the Notes are incompatible with the needs, characteristics and objectives of clients which are retail clients (as defined in Regulation (EU) No. 2017/565 as it forms part of the domestic law of the UK by virtue of the EUWA). Any person subsequently offering, selling or recommending the Notes (a “distributor”) should take into consideration the manufacturers’ target market assessment; however, a distributor subject to the FCA Handbook Product Intervention and Product Governance Sourcebook is responsible for undertaking its own target market assessment in respect of the Notes (by either adopting or refining the manufacturers’ target market assessment) and determining appropriate distribution channels.

S-vi

SUMMARY

The following is a summary of this prospectus supplement and should be read as an introduction to, and in conjunction with, the remainder of this prospectus supplement, the accompanying prospectus and any documents incorporated by reference therein. You should base your investment decision on a consideration of this prospectus supplement, the accompanying prospectus and any documents incorporated by reference therein, as a whole. Words and expressions used in this summary and not defined herein shall have the meanings ascribed to them in “Description of the Notes” in this prospectus supplement and in “Description of Debt Securities” in the accompanying prospectus.

The Issuer

Banco Santander is the parent bank of the Group. The Group operates principally in Spain, the United Kingdom, other European countries, Brazil and other Latin American countries and the United States, offering a wide range of financial products. In Latin America, the Group has majority shareholdings in banks in Argentina, Brazil, Chile, Mexico, Peru, Colombia and Uruguay.

Banco Santander was established on March 21, 1857 and incorporated in its present form by a public deed executed in Santander, Spain, on January 14, 1875. Banco Santander is incorporated under, and governed by, the laws of the Kingdom of Spain as a company with unlimited duration and with limited liability (sociedad anónima).

Banco Santander conducts business under the commercial name “Santander.” The Group’s principal corporate offices are located in Ciudad Grupo Santander, Avenida de Cantabria s/n, 28660 Boadilla del Monte, Madrid, Spain, and its telephone number is (011) 34-91-259-6520.

S-1

The Offering

Issuer

Banco Santander, S.A.

2029 Floating Rate Notes

$ aggregate principal amount of Senior Non Preferred Floating Rate Notes due 2029.

2029 Fixed Rate Notes

$ aggregate principal amount of % Senior Non Preferred Fixed Rate Notes due 2029.

2031 Fixed Rate Notes

$ aggregate principal amount of % Senior Non Preferred Fixed Rate Notes due 2031.

2036 Fixed Rate Notes

$ aggregate principal amount of % Senior Non Preferred Fixed Rate Notes due 2036.

Issue Date

April , 2026.

Maturity Date

The 2029 Floating Rate Notes will mature on April , 2029 (the “2029 Floating Rate Notes Maturity Date”).

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SEC Form 424B5 filed by Banco Santander S.A. Sponsored ADR (Spain)