Kerber became the first German to win the Paris tournament since Steffi Graf in 1995, clinching victory with a forehand winner.
"I knew I had nothing to lose," Kerber said. "She was the favorite today."
After a first set with four service breaks, Kerber capitalized on an unforced error from Bartoli to take a 4-1 lead in the tiebreaker. She clinched the opening set when the Frenchwoman's backhand sailed long.
Kerber then whipped a forehand cross-court winner to break for a 2-0 lead in the next set. But the U.S. Open semifinalist failed to serve out the match at 5-3, and Bartoli won five straight games to even the match with an ace.
"I got a little bit nervous," Kerber said about being 5-2 up in the second set.
Kerber jumped to a 4-0 lead in the final set. Bartoli saved four match points at 5-2 but Kerber converted her fifth match point in the next game.
"I made a few tactical mistakes in the beginning of the third set that cost me," Bartoli said. "I lost the match in those first four games."
Kerber beat top-10 players for the first time in her career this week. As well as edging the 7th-ranked Bartoli, she also ousted third-ranked Maria Sharapova in the quarterfinals Friday.
"That means a lot to me," Kerber said. "Now I believe in me and I know that I can play with them."
Kerber said things clicked for her when she reached the semifinals of the U.S. Open last year.
"From that moment, I was feeling without pressure," Kerber said. "I began to believe in me and in my game. I'm for sure a little bit stronger mentally. I'm a little bit fitter than one year ago."
Bartoli was in tears at the trophy presentation.
"Physically, it was extremely intense," Bartoli said. "I really dug deep to rally. I consumed so much energy and adrenaline that I just let everything go after the match."