The Île de Sein is a French island in the Atlantic Ocean, off Finistère, eight kilometres from the Pointe du Raz, from which it is separated by the Raz de Sein. Its Breton name is Enez Sun. The island, with its neighbouring islets, forms the commune of Île-de-Sein in the Finistère department of Brittany in north-western France.
Lying on the sea routes going south from the English Channel, Sein is well known for the dangers of its waters, the Chaussée de Sein, a vast zone of reefs stretching more than 30 miles from east to west, requiring numerous lighthouses, beacons, and buoys. In the past, it was also known for its wreckers.