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EXHIBIT 12.1

CERTIFICATION

I, Jonathan Cherry, certify that:

|1. | I have reviewed this annual report on Form 20-F of
      PolyMet Mining Corp. (the Company);                                                                                                                                                                                                                                                       
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|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;                   
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|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 Company as of, and for, the periods presented in this
      report;                                         
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|4. | The Companys other certifying officer 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 Company 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 Company, 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
controls 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 Company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 Company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 Companys internal control over
financial reporting.

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

|Date: April 22, 2013 | /s/
      Jonathan Cherry                                
|-------------------- | ---------------------------------------------------------
|                     | Name: Jonathan Cherry                                    
|                     | Title: Chief Executive Officer (Principal
      Executive
|                     | Officer)                                                 

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