Le roi d'Ys (The King of 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. The opera was premiered on 7 May 1888 by the Opéra-Comique at the Théâtre Lyrique on the Place du Châtelet in Paris. Apart from the overture, the most famous piece in the opera is the tenor's aubade in act 3, "Vainement, ma bien-aimée" ("In vain, my beloved"). Lalo was known outside France primarily for other work, but within France Le roi d'Ys was his most recognized work. His first version of the opera was widely rejected during the 1870s, but the revised work met with great success the following decade, becoming his most successful work for the stage.

Namouna is a ballet in two acts and three scenes, with music by Edouard Lalo, choreographed by Lucien Petipa and premiered in Paris in 1882.