Denison Announces Closing of Transaction with Skyharbour and Formation of Four Prospective Exploration Joint Ventures Proximal to Wheeler River

Denison Mines Corp.

1100 – 40 University Ave

Toronto, ON M5J 1T1

www.denisonmines.com

PRESS RELEASE

Denison Announces Closing of Transaction with Skyharbour and Formation of

Four Prospective Exploration Joint Ventures Proximal to Wheeler River

Toronto, ON – December 17, 2025. Denison Mines Corp. (“Denison” or the “Company”) (TSX: DML;

NYSE American: DNN) is pleased to announce that it has closed the previously announced transaction (the

“Transaction”) with Skyharbour Resources Ltd. (“Skyharbour”) (TSX-V:SYH) (OTCQX: SYHBF),

(Frankfurt:SC1P) and the formation of four joint ventures consisting of claims previously comprising

Skyharbour’s Russell Lake Uranium Project (“Russell”), which is located directly adjacent to Denison’s

flagship Wheeler River Project (“Wheeler River”).

The new joint ventures are designed to drive collaboration between Denison and Skyharbour’s technical

teams and to accelerate the evaluation of the prospective exploration ground adjacent to and proximal to

Wheeler River. Denison will serve as the operator for the Wheeler North and Wheeler River Inliers joint

ventures, with ownership interests of 49% and 70% respectively. Skyharbour will serve as the operator for

the Russell Lake (or “RL”) and Getty East joint ventures, with Denison having ownership interests of 20%

and 30%, respectively.

In addition, Denison and Skyharbour have entered into option agreements (the “Earn-In Option

Agreements”), which allow Denison to increase its ownership interest in each of the Wheeler North and

Getty East joint ventures to up to 70%.

About Denison

Denison is a leading uranium mining, development, and exploration company with interests focused in the

Athabasca Basin region of northern Saskatchewan, Canada. Denison has an effective 95% interest in its

flagship Wheeler River Uranium Project, which is the largest undeveloped uranium project in the

infrastructure rich eastern portion of the Athabasca Basin region of northern Saskatchewan.

In mid-2023, the Phoenix feasibility study was completed for the Phoenix deposit as an ISR mining

operation, and an update to the previously prepared 2018 Pre-Feasibility Study ('PFS') was completed for

Wheeler River's Gryphon deposit as a conventional underground mining operation. Based on the respective

studies, both deposits have the potential to be competitive with the lowest cost uranium mining operations

in the world. Permitting efforts for the planned Phoenix ISR operation commenced in 2019 and are nearing

completion with approval in July 2025 of the project's EA by the Province of Saskatchewan and conclusion

in December 2025 of the Canadian Nuclear Safety Commission Public Hearing for Federal approval of the

EA and project construction license.

Denison's interests in Saskatchewan also include a 22.5% ownership interest in the McClean Lake Joint

Venture ('MLJV'), which includes unmined uranium deposits (with mining at McClean North deposit via the

MLJV's SABRE mining method having commenced in July 2025 using the MLJV's SABRE mining method)

and the McClean Lake uranium mill (currently utilizing a portion of its licensed capacity to process the ore

from the Cigar Lake mine under a toll milling agreement), plus a 25.17% interest in the Midwest Joint

Venture Midwest Main and Midwest A deposits, and a 70.55% interest in the Tthe Heldeth Túé ('THT') and

Huskie deposits on the Waterbury Lake Property. The Midwest Main, Midwest A, THT and Huskie deposits

are located within 20 kilometres of the McClean Lake mill. Taken together, Denison has direct ownership

interests in properties covering ~457,000 hectares in the Athabasca Basin region.

Additionally, through its 50% ownership of JCU (Canada) Exploration Company, Limited ('JCU'), Denison

holds interests in various uranium project joint ventures in Canada, including the Millennium project (JCU,

30.099%), the Kiggavik project (JCU, 33.8118%) and Christie Lake (JCU, 34.4508%).

In 2024, Denison celebrated its 70th year in uranium mining, exploration, and development, which began

in 1954 with Denison's first acquisition of mining claims in the Elliot Lake region of northern Ontario.

For more information, please contact

David Cates (416) 979-1991 ext. 362

President and Chief Executive Officer

Geoff Smith (416) 979-1991 ext. 358

Vice President Corporate Development & Commercial

Follow Denison on X (formerly Twitter) @DenisonMinesCo

About Skyharbour

Skyharbour holds an extensive portfolio of uranium exploration projects in Canada's Athabasca Basin and

is well positioned to benefit from improving uranium market fundamentals with interest in thirty-seven

projects covering over 616,000 hectares (over 1.5 million acres) of land. Skyharbour has acquired from

Denison, a large strategic shareholder of Skyharbour, a 100% interest in the Moore Uranium Project, which

is located 15 kilometres east of Denison's Wheeler River project and 39 kilometres south of Cameco's

McArthur River uranium mine. Moore is an advanced-stage uranium exploration property with high-grade

uranium mineralization in several zones at the Maverick Corridor. Adjacent to the Moore Project is the

Russell Lake Uranium Project, which hosts widespread uranium mineralization in drill intercepts over a

large property area with exploration upside potential. Skyharbour is actively advancing these projects

through exploration and drilling programs.

Skyharbour also has joint ventures with industry leaders Denison, Orano Canada Inc., Azincourt Energy,

and Thunderbird Resources at the Russell, Preston, East Preston, and Hook Lake Projects, respectively.

Skyharbour also has several active earn-in option partners, including CSE-listed Basin Uranium Corp. at

the Mann Lake Uranium Project; TSX-V listed North Shore Uranium at the Falcon Project; UraEx Resources

at the South Dufferin and Bolt Projects; Hatchet Uranium at the Highway Project; CSE-listed Mustang

Energy at the 914W Project; and TSX-V listed Terra Clean Energy at the South Falcon East Project.

Cautionary Statement Regarding Forward-Looking Statements

Certain information contained in this news release constitutes ‘forward-looking information’, within the meaning of the applicable United

States and Canadian legislation, concerning the business, operations and financial performance and condition of Denison. Generally,

these forward-looking statements can be identified by the use of forward- looking terminology such as ‘potential’, ‘plans’, ‘expects’,

‘budget’, ‘scheduled’, ‘estimates’, ‘forecasts’, ‘intends’, ‘anticipates’, or ‘believes’, or the negatives and/or variations of such words and

phrases, or state that certain actions, events or results ‘may’, ‘could’, ‘would’, ‘might’ or ‘will’ ‘be taken’, ‘occur’ or ‘be achieved’.

In particular, this news release contains forward- looking information pertaining to Denison's current intentions , objectives and

commitments with respect to the Transaction and the expected benefits thereof; the Company’s exploration, development and

expansion plans and objectives; and expectations regarding its joint venture ownership interests and the continuity of its agreements

with its partners and third parties.

Forward looking statements are based on the opinions and estimates of management as of the date such statements are made, and

they are subject to known and unknown risks, uncertainties and other factors that may cause the actual results, level of acti vity,

performance or achievements of Denison to be materially different from those expressed or implied by such forward- looking

statements. For example, the parties to the Option Agreement may not complete the option phases as described and/or the exploration

objectives may not be achieved. In addition, Denison may decide or otherwise be required to discontinue testing, evaluation and other

work on the Company’s other properties if it is unable to maintain or otherwise secure the necessary resources (such as testing

facilities, capital funding, joint venture approvals, regulatory approvals, etc.). Denison believes that the expectations reflected in this

forward-looking information are reasonable but no assurance can be given that these expectations will prove to be accurate and

results may differ materially from those anticipated in this forward- looking information. For a discussion in respect of risks and other

factors that could influence forward-looking events, please refer to the factors discussed in Denison’s Annual Information Form dated

March 28, 2025 under the heading ‘Risk Factors’ or in subsequent quarterly financial reports. These factors are not, and should not

be construed as being, exhaustive.

Accordingly, readers should not place undue reliance on forward- looking statements. The forward- looking information contained in

this news release is expressly qualified by this cautionary statement. Any forward-looking information and the assumptions made with

respect thereto speaks only as of the date of this news release. Denison does not undertake any obligation to publicly update or revise

any forward-looking information after the date of this news release to conform such information to actual results or to changes in

Denison's expectations except as otherwise required by applicable legislation.