St Mary's Island may refer to:
Places England St Mary's Island, Kent, in Medway St Mary's Island, River Thames, in the county of Berkshire St Mary's Island (Tyne and Wear), in the metropolitan county of Tyne and Wear St Mary's, Isles of Scilly, in the county of Cornwall
India St. Mary's Islands, in the state of Karnataka
Isle of Man St Mary's Isle, Douglas Bay
Madagascar Île Sainte-Marie, also known as St Mary's Island in English, in the region of Analanjirofo
The Gambia Another name for Banjul Island, in Banjul division