Company: SGBAF
Filing Date: 2025-09-26
Form Type: 6-K
Source: 0001193125-25-220450
Chunk: 0

Company: SES S.A.
Filing Date: 2025-09-26
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-16of the

Securities Exchange Act of 1934

Date: September 26, 2025

Commission File Number: 333-286828

SES

(Translation of registrant’s name into English)

Château de Betzdorf

L-6815Betzdorf

Grand Duchy of Luxembourg

(Address of principal executive offices)

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 ☐

Explanatory Note On September 26, 2025, SES (the “Company”) issued a press release announcing an interim dividend to be paid in October 2025. A copy of the press release is furnished as Exhibit 99.1 to this Report on Form 6-Kand incorporated by reference herein. On September 26, 2025, the Company issued a press release announcing the appointment of Joseph Cohen to the Company’s Board of Directors and the departure of Kaj-Erik Relander from the Company’s Board of Directors. A copy of the press release is furnished as Exhibit 99.2 to this Report on Form 6-Kand incorporated by reference herein. EXHIBIT INDEX

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.

| Date: September 26, 2025 |     | SES          
 (registrant) 
 By:          |     | /s/ Aaron Shourie   |
|:-------------------------|:----|:-------------|:----|:--------------------|
|                          |     | Name:        |     | Aaron Shourie       |
|                          |     | Title:       |     | Chief Legal Officer |