After a rain-delayed start, the Russian appeared to be in a hurry to get off court again and won 14 of the first 15 points to race into a 4-0 lead before clinching an easy first set.
The battling Australian had no answer to Sharapova's range of shots in the second set and Russian clinched her 23rd career title and first in Rome with another devastating forehand winner.
"I'm so excited to be champion of this tournament, it's something I've dreamed about," Sharapova said while clutching the trophy. "This is just the beginning of many things to come."
Stosur, who had committed 14 unforced errors to Sharapova's five in the first set, began the second set with two more wayward forehands as the seventh-seed broke yet again.
But the Australian, runner-up at Roland Garros last year, got lucky in the very next game to break back as her mishit forehand dropped on the baseline to give her a 2-1 lead.
Sharapova, who needs the French Open title to complete a career grand slam, kept her focus to take the next three games against an opponent she had beaten in all seven of their previous meetings.
The Russian negated sixth seed Stosur's wayward power game with some canny placements and finished off a long sixth game with two rasping forehands.
The Australian, who sat through the changeover with her head bowed low, finally managed to hold on to two consecutive service games but she was powerless to stop Sharapova from lifting a second claycourt title in Europe.
