The 1977 Prix de l'Arc de Triomphe was a horse race held at Longchamp on Sunday 2 October 1977. It was the 56th running of the Prix de l'Arc de Triomphe. The winner was Alleged, a three-year-old colt trained in Ireland by Vincent O'Brien and ridden by Lester Piggott. Alleged became the fourth Irish-trained horse to win the race twice after Ballymoss (also trained by O'Brien) in 1957 and Levmoss in 1970. Piggott had previously won the race on Rheingold in 1973.