Solemia, ridden by Olivier Peslier, won the Prix de l'Arc de Triomphe at Longchamp, pipping Japanese horse Orfevre at the post.
It was a fourth win in the famous race for the French jockey, who won three consecutive Arcs between 1996-98, on Helissio, Peintre Celebre and Sagamix.
Orfevre (5-1), Japan's 2011 horse of the year, looked certain to claim his country's first victory.
But the 33-1 outsider's sweeping run in the home straight secured the win.
The Aidan O'Brien-trained Ernest Hemingway led for much of the race but stablemate Camelot, ridden by Frankie Dettori, failed to live up to its billing as one of the favourites, finishing seventh on a rain-softened course.
Masterstroke (11-1) came third with Haya Landa (150-1) fourth.
"It's the Arc," said four-year-old filly Solemia's trainer Carlos Laffon-Parias. "I thought we were beaten in the straight, then I saw Orfevre stopping.
"Sometimes the jockey can make the difference and Olivier Peslier is the one who won the race."
Peslier, 39, said: "It's fantastic, for the yard, for everyone."
O'Brien said of Camelot: "It was very sporting of the lads to let him run, but he wants fast ground.
"He's something for us to look forward to next season, as he will remain in training. Hopefully he has a good winter and he could be something unbelievable next year."
Dettori said: "It was just a long season took its toll."