Le roi d'Ys is an opera in three acts and five tableaux by the French composer Édouard Lalo, to a libretto by Édouard Blau, based on the old Breton legend of the drowned city of Ys. That city was, according to the legend, the capital of the kingdom of Cornouaille.

Édouard-Victoire-Antoine Lalo was a French composer. His most celebrated piece is the Symphonie Espagnole, a five-movement concerto for violin and orchestra that remains a popular work in the standard repertoire.