The ninth-seeded Radwanska was down 2-0 at Ariake Colosseum but stormed back to win the first set on the back of Zvonareva's 22 unforced errors.
"I was a little bit nervous at first," Radwanska said. "Those first two games went by so fast, I was just trying to do my best. I'm just happy that I woke up quickly and won the first set."
Zvonareva overcame a 5-1 deficit against Petra Kvitova on Friday in the semifinals, but couldn't stage another turnaround in the final.
Radwanska broke Zvonareva twice to take a 5-2 lead. Zvonareva held to win the eighth game but Radwanska closed out the first set with a forehand to the corner that Zvonareva hit long.
The fourth-seeded Russian struggled with her serve, double faulting five times. She broke Radwanska to win the sixth game of the second set to make it 4-2, but any chance of a comeback was dashed when Radwanska broke back in the next game.
"I felt like I started out well and then something happened," Zvonareva said. "It was not me on the court anymore. I will think about it and try to figure out what happened but it's one of those things that can happen to anyone."
Radwanska won her first singles title in three years when she beat Zvonareva in Carlsbad, Calif., in August. The Polish star also beat the Russian player in the third round at the Rogers Cup in Toronto the week after Carlsbad.
With the tournament victory, Radwanska still has a chance to reach the WTA Championships in Turkey.
"Before here, I didn't have really big chances because I was around 14th," Radwanska said. "I just knew that I had to play really well here and in Beijing just to have a chance. Now, for sure my chances are much bigger to go so I will fight to the end."
The top eight singles players qualify for the WTA Championships. Radwanska is projected to move up to 12th in the rankings after her victory.