Therasia, also known as Thirasía, is an island in the volcanic island group of Santorini in the Greek Cyclades. It lies north-west of Nea Kameni, a small island formed in recent centuries by volcanic activity and thus marking the centre of the island group. Therasia is the second largest island of the group, the largest by far being Thera.
Therasia has a land area of 9.299 square kilometres and its population was 319 inhabitants at the 2011 census. It is part of the municipal unit of Oia.
Thera and Therasia were separated by the Thera eruption.
Coordinates 36°27'2.765" N 25°20'47.476" E