Thurston Island is an ice-covered, glacially dissected island, long, wide and in area, lying a short way off the northwest end of Ellsworth Land, Antarctica. It is the third largest island of Antarctica, after Alexander Island and Berkner Island. Thurston Island is separated from the mainland by Peacock Sound, which is occupied by the western portion of Abbot Ice Shelf. It divides Bellingshausen Sea to the east from Amundsen Sea to the west.