After a three-hour rain delay, the seventh-seeded Sharapova won the opening four games, then cruised from there to follow up her victory over top-ranked Caroline Wozniacki in the semifinals.
This tournament is a key warmup for the French Open, the clay-court Grand Slam that begins in one week, and Sharapova will now clearly be one of the favorites in Paris.
Sharapova is a three-time Grand Slam winner, and Roland Garros remains the only major title she hasn't won.
"This is just the beginning of many things to come. This is just the start of everything," Sharapova said during the trophy presentation.
In the men's final, top-ranked Rafael Nadal was set to resume his budding rivalry with No. 2 Novak Djokovic. Djokovic beat Nadal in the three previous finals the two have played this year, including his first victory on clay over the Spaniard in Madrid a week ago.
Nadal has won this clay-court tournament five of the last six years, while Djokovic is riding a 38-match winning streak.
Stosur hadn't reached a final since losing to Francesca Schiavone at Roland Garros last year, and the Australian is now 0-8 in her career against Sharapova.
A former No. 1, Sharapova had been struggling to regain her top form since undergoing right shoulder surgery in October 2008 -- which caused her to miss 10 months.
The Russian won her last title in Strasbourg, France, a year ago.
With her fiance and New Jersey Nets guard Sasha Vujacic watching from the stands, Sharapova didn't drop a set all week, and she needed just 1 hour, 23 minutes to beat Stosur.