After dropping the first set 4-6, the event's eighth seed won the next two by scores of 6-3, 6-2.
It was her ninth singles title on the Women's Professional Tennis Tour and first this season.
After Kuznetsova captured the first set and won the first game of the second set, Dementieva won four straight games to take control. She broke serve in the ninth game to send the match to a decisive third set.
A break right after her own service was broken led to Dementieva improving to 9-11 lifetime in finals.
"I think I was playing wrong tactically in the first set," Dementieva said. "I was hitting the ball too short."
Kuznetsova had won their previous three meetings.