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 km to the north-east, 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 metres by 150 metres, and 45 metres high. It is composed of granite. It is in the commune of Genêts.
In the 11th century, two monks from Mont Saint-Michel were hermits on Tombelaine. In 1137 Bernard du Bec founded a priory on the island, and it became a place of pilgrimage.
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..