Osmussaar is an Estonian island situated in the mouth of the Gulf of Finland in the Baltic Sea, 7.5 km off the Estonian mainland. Administratively the island is part of Noarootsi Parish in Lääne County. Its area is .
Before the Soviet Union occupied Estonia during World War II, around 130 people, mainly Estonian Swedes, lived on the island. The continuous settlement of Swedes on Osmussaar had dated back for centuries. Currently Osmussaar has only two permanent inhabitants and the island is a nature reserve.