The 25-year-old, whose last tournament triumph came in the US Open last September, prevailed 7-6(5) 6-1 in front of her home crowd.
Both players struggled to hold serve in a tense opening set, with eight breaks in all, but it was Stephens who found her rhythm quickest.
After breaking her opponent in the 12th game, the Florida native took her fourth set point in the tie-break to edge in front in the contest.
Ostapenko struggled to cope in the second set in what was the first ever meeting between the pair, despite immediately breaking Stephens in the opening game.
Stephens took the next six games on the spin to win her sixth WTA title, while also maintaining a perfect record in finals.