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EXHIBIT 12.1

CERTIFICATION

I, Joseph Scipioni, certify that:

I have reviewed this annual report on Form 20-F/A of PolyMet
Mining Corp.;

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;

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;

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.

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):

(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 17, 2009 | /s/ Joseph Scipioni                                      
|-------------------- | ---------------------------------------------------------
|                     | Name: Joseph Scipioni                                    
|                     | Title: Chief Executive Officer (Principal
      Executive
|                     | Officer)                                                 

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