The 3-year-old Motivator filly circled the field around the turn and burst away from weakening pacesetter Joshua Tree and 15 other challengers at the top of the straight. Lengthening her lead with every stride, she crossed the finish line five lengths clear of Japanese star Orfevre, who outfought Prix du Jockey Club (Fr-I, French Derby) winner Intello to be the runner-up for a second straight year.
Treve started from difficult post 15 and finished the 2,400-meter (about 1 1/2-mile) race, the continent's premier middle-distance event, in 2.32.04 over soft going.
"I tried to ride this filly as bested suited her," said jockey Thierry Jarnet, who inherited the mount when jockey Frankie Dettori broke his ankle in a fall earlier in the week.
Treve ran most of the race on the outside. She hardly seemed in contention halfway through, stuck in 14th place. But she made her move with about 2 1/2 furlongs to go, taking the lead and pulling away in the final straightaway.
"It was magical today because I had the luck to ride a great champion,'' the 46-year-old Jarnet said.
Jarnet won the Arc two times previously, with Subotica in 1992 and Canegie in 1994.
Intello finished a neck behind favored Orfevre and bested Tokyo Yushun (Jpn-I, Japanese Derby) winner Kizuna by two lengths for third.
Al Kazeem, a three-time group I winner this year, finished sixth, Epsom Derby (Eng-I) winner Ruler of The World was seventh, early Arc favorite Flintshire was eighth, and Ladbrokes St. Leger (Eng-I) victor Leading Light checked in 12th.
Treve is undefeated in five career starts that include the Prix de Diane (Fr-I, French Oaks) in June and the Prix Vermeille (Fr-I) Sept. 15 in her previous start. She is the sixth filly to complete the Vermeille/Arc double the same year and first since Zarkava in 2008.
"She did something completely unbelievable," said Christophe Soumillon, Orfevre's rider. "I don't think today Treve was beatable."
Treve is trained by Criquette Head-Maarek for Qatar Sheikh Joaan Al Thani. She was bred in France by the Head family's Haras de Quesnay, which stands Motivator, an 11-year-old son of Montjeu. Head-Maarek enjoyed a second Arc triumph after winning in 1979 with Three Troikas.
"Until now, we didn't ask much from her,'' said Christiane Head-Maarek of Treve. "Today, she really showed she's a champion because the race didn't go well for her. I thought she had six gears but today she put the seventh gear on. And when she left, she won so easily.''
Japan hoped Orfevre would deliver the country's first Arc victory after narrowly losing to another French-based filly, Solemia, last year, but Treve proved too strong.
"I was surprised by Treve's performance,'' Orfevre's trainer Yasutoshi Ikee said.
Treve's dam is the winning Anabaa mare Trevise, a full sister to United States stakes winner Tsigane, who was grade I-placed. Her second dam, Trevillari, is a full sister to French group I winner Treble. Fourth dam Trillion was champion in the United States and Europe. Treve is a half sister to French group II-placed Trois Rois.