Petrova, the No. 17 seed, had 11 aces in a 6-0, 1-6, 6-3 win.
She roared through the first set, losing just four points on serve, but Radwanska, who was seeded third, answered by saving four break points in the second set. Both players had won 44 points on the day at that point.
They both played a strong third set but Radwanska had consecutive double faults -- her only doubles of the match -- to give Petrova a break point in the eighth game. She converted that and then held serve for the victory.
Petrova defeated three Top 10 players -- world No. 3 Radwanska, No. 9 Samantha Stosur in the semifinals and No. 6 Sara Errani in the quarterfinals -- in Tokyo en route to her 12th WTA title and second this year. Radwanska, in addition to looking to repeat in Tokyo, was trying for her fourth championship in 2012.
Petrova collected $385,000 for the title. The runner-up check for Radwanska totaled $192,000.