Parmentier, the eighth seed, withstood five aces from Pegula and claimed half of the American's second service points to win her fourth career title.
Her other victory this year came at Istanbul in April.
Parmentier, who is 4-0 in career finals, also saved three of four break points against her, while breaking Pegula four times.
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Pegula, seeking her first title, broke early for a 3-1 lead in the first set before Parmentier battled back.
With the score 5-5, Pegula saved two break points, the second with an ace before Parmentier converted a third with a forehand winner to go up 6-5.
Serving for the set, the Frenchwoman came back from a 0-30 deficit and closed out the set four points later on another forehand winner.
Parmentier opened up a 4-0 lead in the second set and served out the match after Pegula had closed to within 5-2.
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