Company: SLFQF
Filing Date: 2025-02-13
Form Type: 6-K
Source: 0001193125-25-026085
Chunk: 0

Company: SUN LIFE FINANCIAL INC
Filing Date: 2025-02-13
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-16

under the Securities Exchange Act of 1934

For the month of February 2025

Commission File Number: 001-15014

### SUN LIFE FINANCIAL INC.
(Translation of registrant’s name into English)

1 York Street, 31st Floor, Toronto, Ontario, M5J 0B6

(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 ☒

Exhibit 99.1 of this Form 6-K is incorporated by reference into the registrant’s registration
statement on Form S-8 (File No. 333-269768).

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: February 13, 2025 |     | Sun Life Financial Inc. 
 (Registrant)            
 By:                     |     | /s/ Tracie Allan                           |
|:------------------------|:----|:------------------------|:----|:-------------------------------------------|
|                         |     |                         |     | Tracie Allan                               |
|                         |     |                         |     | Vice-President & Associate General Counsel |