It was the fourth Arc victory for the popular Dettori, who also won the English Derby earlier this summer on the John Gosden-trained three-year-old colt.
Golden Horn hit the front early at the famous Parisian racecourse with Treve, the winner for the past two years, mounting a strong trademark challenge under Thierry Jarvet.
But it petered out with Flintshire finishing runner-up for the second straight year and French Derby winner New Bay in third.
Five-year-old Treve, trained by Criquette Head-Maarek, finished a disappointing fourth in Europe's most prestigious horse race.
She confirmed that the horse is now likely headed for retirement, but said defeat against such high-class opposition was no disgrace and on fast ground that did not suit her.
"Before the race I was scared of Golden Horn, New Bay and Flintshire and they have filled the frame. In racing there are some you win and some in which you get beat, that's the game we are in," she said.
Golden Horn -- described by Dettori as "probably the best horse I have ridden" -- will be retired to stud next year, but owner Anthony Oppenheimer may target the Breeders' Cup or the Champion Stakes at Ascot for a finale.
"He was magnificently ridden and has just a wonderful trainer in John Gosden," he said. "As for the future, we'll let the horse tell us whether we run again."
If Golden Horn was to contest the Breeders Cup Classic at Keeneland in the United States on October 31 it would set up an intriguing clash with American Pharoah, the winner of Triple Crown this season.
"This might have taken a bit out of him, though, so we'll see," added Oppenheimer.