The 1978 Prix de l'Arc de Triomphe was a horse race held at Longchamp on Sunday 1 October 1978.
The winner was Alleged, a four-year-old colt trained in Ireland by Vincent O'Brien and ridden by Lester Piggott.
Alleged, the 14/10 favourite, defeated the mare Trillion by two lengths, with Dancing Maid a further two lengths back in third.
Alleged became the sixth horse to win the race twice after Ksar, Motrico, Corrida, Tantieme and Ribot.
* Abbreviations: ns = nose; shd = short-head; hd = head; snk = short neck; nk = neck Further details of the winner, Alleged.