Report of Foreign Issuer Pursuant to Rule 13a-16 or 15d-16 (6-k)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER PURSUANT TO RULE 13a-16 OR 15d-16

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Date: September 28, 2021

Commission File Number: 001-33414

Denison Mines Corp.

(Name of registrant)

1100-40 University Avenue

Toronto, Ontario

M5J 1T1 Canada

(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

x 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): ¨

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): ¨

INCORPORATION BY REFERENCE

Exhibit 99.2 to this report on Form 6-K is hereby incorporated by reference as an Exhibit to the Registration Statement on Form F-10 of Denison Mines Corp. (File No. 333-258939), as amended and supplemented.

SIGNATURES

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.

Denison Mines Corp.