Found, ridden by Ryan Moore, won the Prix de l'Arc de Triomphe at Chantilly, leading home a one-two-three for Irish trainer Aidan O'Brien.
Found (6-1) upped the pace in the final furlongs to power past pacemaker Vedevani and surge to victory.
Highland Reel (20-1) was second under Seamie Heffernan, while Frankie Dettori - trying to win his fifth Arc - was third on Order of St George (14-1),
"I couldn't dream this would happen," said O'Brien.
"It's a privilege to be here and be part of this."
Pre-race favourite Postponed, ridden by Italian Andrea Atzeni, finished fifth, while Japan's Makahiki struggled to make an impact.
BBC racing correspondent Cornelius Lysaght
During the 20 or so years he's been training, Aidan O'Brien has achieved many great feats - even 1-2-3s in Classics - but to saddle a trio in Europe's top flat-racing prize and fill the first three places has to rank as quite likely best of all.
Some might dispute it, but the Arc is the outstanding race staged in Europe and very hard to win.
Even O'Brien took years to be successful for the first time (with Dylan Thomas in 2007) and now he gets this result.
Famously modest, he said afterwards: "How could you say anything is higher than that?"
Jockey Ryan Moore compared the result to Frankie Dettori's Magnificent Seven at Ascot in 1996 or Michael Dickinson's 1-2-3-4-5 in the 1983 Cheltenham Gold Cup.
Great stuff for the trainer and for the Coolmore racing and thoroughbred breeding empire, of course, and for female horses which have won now six out of nine Arcs.