The 1978 Prix de l'Arc de Triomphe was a horse race held at Longchamp on Sunday 1 October 1978. It was the 57th running of the Prix de l'Arc de Triomphe.
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. The winning time was 2:36.1.
<span style="font-size:85%">* Abbreviations: ns = nose; shd = short-head; hd = head; snk = short neck; nk = neck</span>
Further details of the winner, Alleged.