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.
On 11 February 1423, in the Hundred Years War, Tombelaine was occupied by the English as a base to attack Mont Saint-Michel. In the 16th-century French religious wars, Gabriel, comte de Montgomery, leader of the Huguenot armies, occupied the island.
In 1666 the marquis de la Chastrière ordered the destruction of the island's fortifications, in case they were again used by the English.
- Map of Mont St Michel Bay, showing crossing routes
- Genets website (in French)
- Notice PA00110408, Base Mérimée
- Sinsoilliez, Robert (2000). Tombelaine, L'îlot de la baie du mont Saint-Michel (in French). p. 192. ISBN 2-84141-157-5..