And for the second time in 2011, she won her semi-final and final on the same day, having had to go down the same route when triumphing at Eastbourne in June in the run-up to Wimbledon.
Wet weather in Osaka meant both semi-finals were held over until Sunday, with second seed Bartoli taking on Germany's Angelique Kerber.
She won that 6-1 7-6 (7/5), while Australian top seed Stosur battled through a tough three-set match against China's Zheng Jie.
Stosur survived, winning 7-6 (7/5) 3-6 6-3, but had little left for the final and suffered a 6-3 6-1 loss, struggling to make an impression against Bartoli's serve.
Bartoli, who also beat Stosur in her Eastbourne semi-final, believes her mental strength was decisive.
She said on wtatennis.com: "Maybe I should play two matches in one day every time!
"They are long days but it's very much mental. I don't think it's a matter of fitness or stamina. It's how much you have left mentally."
Bartoli still has a slim chance of reaching the end-of-season WTA Championships but must win the Kremlin Cup title in Moscow in this coming week and hope Poland's Agnieszka Radwanska loses her opening match. Both Bartoli and Radwanska have first-round byes.