Company: BCDRF
Filing Date: 2025-01-17
Form Type: 6-K
Source: 0001193125-25-008410
Chunk: 0

Company: Banco Santander, S.A.
Filing Date: 2025-01-17
Form: 6-K
Chunk 0
---
### UNITED STATES

### SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 6-K

### REPORT OF
FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16OR 15d-16UNDER

THE SECURITIES EXCHANGE ACT OF 1934

For the month of January, 2025.

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 ☐

Banco Santander, S.A. EXPLANATORY NOTE In connection with the issuance by Banco Santander, S.A. of $1,000,000,000 aggregate principal amount of 5.565% Senior Non Preferred Fixed Rate Notes due 2030 and $1,000,000,000 aggregate principal amount of 6.033% Senior Non Preferred Fixed Rate Notes due 2035, Banco Santander, S.A. is filing the following documents solely for incorporation into the Registration Statement on Form F-3(File No. 333-271955): Exhibit List

| Exhibit No. 
 1.1         
 4.1         |     | Description                                                                                                                                                                                                      
 Underwriting Agreement dated as of January 7, 2025                                                                                                                                                               
 Second Supplemental Indenture, dated as of January 17, 2025, to the Senior Non Preferred Debt Securities Indenture, dated as of March                                                                            
 14, 2024, among Banco Santander, S.A., as Issuer, The Bank of New York Mellon, London Branch, as Trustee and Principal Paying Agent, and The Bank of New York Mellon SA/NV, Luxembourg Branch, as Registrar      |
|:------------|:----|:-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 4.2         |     | Form of Global Note for the 5.565% Senior Non Preferred Fixed Rate Notes due 2030 (included in Exhibit A of the Second Supplemental Indenture to the Senior Non Preferred Debt Securities Indenture (Exhibit 4.1 
 to this filing))                                                                                                                                                                                                 |
| 4.3         |     |