Sein can refer to:
Places Île-de-Sein, an island and commune in Brittany, France Raz de Sein, a stretch of water in Brittany, France
People Given name Sein Aye, birthname of Sitt Nyein Aye (born 1956), Burmese artist Sein Hlaing (1918-2010), Burmese footballer and coach Sein Lwin (1923-2004), Burmese politician, 6th President of the Union of Burma Po Sein (1882-1954), Burmese actor, singer and dancer U Sein Than, Burmese land reform activist Thein Sein (born 1945), Burmese politician and military commander, 8th President of Myanmar
Surname Mai Šein (born 1946), Estonian architect
Fictional characters Sein, character in Magical Girl Lyrical Nanoha Strikers
Other SEIN: Société d'encouragement pour l'industrie nationale, organization established in 1801 to encourage French industry