Marly-le-Roi (märlēˈ-lə-rwä) [key], town (1990 pop. 16,775), Yvelines dept., N France, on the Seine River near Versailles. Nearby is the hamlet of Marly-la-Machine, where in 1682 a huge hydraulic engine, the machine de Marly, was built to supply the fountains of Versailles. Considered one of the wonders of the world, the engine was in use until 1804. In the town is a church (1689) built by J. H. Mansart.