Exhibit 99.5

CERTIFICATION

REQUIRED
BY RULE 13a-14(a) OR RULE 15d-14(a)

I, David D. Cates, certify that:

1. I have reviewed this annual report on Form 40-F of Denison Mines Corp.;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the
financial condition, results of operations and cash flows of the issuer as of, and for, the periods presented in this report;

4. The issuers other
certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act
Rules 13a-15(f) and 15d-15(f)) for the issuer and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure
controls and procedures to be designed under our supervision, to ensure that material information relating to the issuer, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in
which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over
financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted
accounting principles.

(c) Evaluated the effectiveness of the issuers disclosure controls and procedures and presented in this
report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this report any change in the issuers internal control over financial reporting that occurred during the period covered
by the annual report that has materially affected, or is reasonably likely to materially affect, the issuers internal control over financial reporting; and

5. The issuers other certifying officer and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the
issuers auditors and the audit committee of the issuers board of directors (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are
reasonably likely to adversely affect the issuers ability to record, process, summarize and report financial information; and

(b)
Any fraud, whether or not material, that involves management or other employees who have a significant role in the issuers internal control over financial reporting.

Date: March 30, 2017



|       |  |                                      
|------ |  | -------------------------------------
|By:    |  | /s/ David D. Cates                   
|Name:  |  | David D. Cates                       
|Title: |  | President and Chief Executive Officer



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CERTIFICATION

REQUIRED BY RULE 13a-14(a) OR RULE 15d-14(a)

I, Gabriel McDonald, certify that:

1. I have reviewed
this annual report on Form 40-F of Denison Mines Corp.;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or
omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the
financial condition, results of operations and cash flows of the issuer as of, and for, the periods presented in this report;

4. The issuers other
certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act
Rules 13a-15(f) and 15d-15(f)) for the issuer and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure
controls and procedures to be designed under our supervision, to ensure that material information relating to the issuer, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in
which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over
financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted
accounting principles.

(c) Evaluated the effectiveness of the issuers disclosure controls and procedures and presented in this
report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this report any change in the issuers internal control over financial reporting that occurred during the period covered
by the annual report that has materially affected, or is reasonably likely to materially affect, the issuers internal control over financial reporting; and

5. The issuers other certifying officer and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the
issuers auditors and the audit committee of the issuers board of directors (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are
reasonably likely to adversely affect the issuers ability to record, process, summarize and report financial information; and

(b)
Any fraud, whether or not material, that involves management or other employees who have a significant role in the issuers internal control over financial reporting.

Date: March 30, 2017



|       |  |                                                    
|------ |  | ---------------------------------------------------
|By:    |  | /s/ Gabriel McDonald                               
|Name:  |  | Gabriel McDonald                                   
|Title: |  | Vice President, Finance and Chief Financial Officer