Radwanska committed only 10 unforced errors to Sharapova's 26 in the first set alone in her 7-5, 6-4 victory, nailing down her ninth career championship in the process.
The victory was the biggest in the career of Radwanska, 23, who also took the Dubai championship this season and improved to 9-2 all-time in WTA finals.
She had advanced to the final with a 6-4, 6-2 pounding of Marion Bartoli in Thursday's semifinal.
Sharapova, meanwhile, was ousted in the Miami final for the second straight year.
The Russian star entered the match with a 7-1 lifetime mark against Radwanska, who earned a first prize of $712,000.