Sainte-Marthe-sur-le-Lac is an off-island suburb of Montreal, in the Canadian province of Quebec, in the Deux-Montagnes Regional County Municipality, 40 km from Montreal. It is crossed from east to west by Route 344, commonly known as Oka Road. The town shares its borders with Deux-Montagnes to the east, Saint-Joseph-du-Lac to the west, the Lake of Two Mountains to the south, and Saint-Eustache to the north.