The Île de Sein [il də sɛ̃] is a Breton island in the Atlantic Ocean, off Finistère, eight kilometres from the Pointe du Raz (raz meaning "water current"), 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. Inhabitants of Île-de-Sein are called in French Sénans.
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. The lighthouses near the reef include Ar Men, La Vieille and Tévennec.
