Tombelaine is a small tidal island off the coast of Normandy in France. It lies a few kilometres north of Mont Saint-Michel. At low tide the island can be reached on foot (with a guide) from the coast of Cotentin, 3.5 kilometres (2.2 mi) to the northeast, and from Mont Saint-Michel. The island lies just to the south of the course of the Sélune river, which has to be forded to access the island from Cotentin. The island is 250 m (820 ft) by 150 m (490 ft), and 45 m (148 ft) high. It is composed of granite. It is in the commune of Genêts.