Therasia is an island in the island group of Santorini in the Greek Cyclades.
Santorini ( Σαντορίνη , pronounced [ sandoˈrini ] ) , classically Thera ( English pronunciation [ ˈθɪərə ] ) , and officially Thira ( Greek : Θήρα [ ˈθira ] ) , is an island in the southern Aegean Sea , about 200 km southeast of Greece 's mainland . It forms the southernmost member of the Cyclades group of islands , with an area of approximately 73 km2 and a 2011 census population of 15,550 . Therasia , also known as Thirasía , is an island in the volcanic island group of Santorini in the Greek Cyclades .