She was behind in the first set, but rattled off points to win it 7-5 before sealing the second set 6-3 despite being 3-1 down.
The fifth seed and world No. 63, adds to her Saint-Malo crown from 2024.
In Romania local favourite Irina-Camelia Begu beat Swiss Jil Teichmann 6-0 7-5 to win the Iasi Open.
It was her first title in three years and Teichmann's first final in nearly four.
The WTA tour now goes to Washington in a hardcourt warm-up for the US Open with a surprise entrant at the DC Open: Venus Williams.
The seven-time major winner has not competed for more than a year.
Her last tournament was in March 2024 at the Miami Open, where she exited in the first round. It's been so long that the official WTA Tour website lists her as "inactive."
"This is very special for me to come back and play tennis," Williams, who turned 45 last month, said. "I think it's a surprise for the fans — and a surprise in general, as I hold my cards tight."
What did she miss the most while away from the tour?
"All the times that I had and, of course, the adrenaline, all those things," Williams said. "Just the pure fun of playing the game, the fun of the challenge, overcoming — when you play, you overcome so many challenges: your opponents, the conditions, a lot of times you have to overcome yourself. Those things are very exciting."
When asked why she's getting back on court, Williams replied: "Why not?"
"I'm just here for now, and who knows?" she added. "Maybe there's more. ... But at the moment, I'm focused just on this.
"I haven't played in a year. There is no doubt I can play tennis, but obviously coming back to play matches, it takes time to get in the swing of things."
With AP
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