The unseeded Pavlyuchenkova took advantage of seven double faults to rally past fellow Russian Sharapova, 4-6, 6-3, 6-4 indoors in Paris.
The former world No. 1 was broken five times in the defeat.
Pavlyuchenkova was coming off another upset win -- she doused fourth-seeded 2012 champ Angelique Kerber Friday. The 22-year-old will be looking for a sixth career championship.
Her finals opponent will be third-seeded Italian Errani, who turned back a challenge from unseeded Alize Cornet of France, 7-6 (7-3), 3-6, 7-6 (7-5) in the day's late semifinal.
Errani, who bested Pavlyuchenkova in their only other meeting, is seeking her eighth career title.