The fifth-seeded Razzano was in her fourth career final, and third this season. The win marked the Frenchwoman's second title on the WTA tour, including last week's Guangzhou International at Guangzhou, China.
Her showing in Tokyo will allow her break into the Top 30 next week for the first time in her career.
"I try to do my best on every point and in every game," Razzano said. "It was difficult to face the match points, but I concentrated and played my best at those times (and) I played simply and aggressively. I'm happy I was able to come back."
As the victor, Razzano earned $25,840.
"She played really well on the big points," said Williams, a six-time major champion. "She kept getting the ball back, and it's always important to do that on those points. It was good tennis."