Thera, Modern Greek Thíra, also called Santorin, or Santoríni, island, southernmost island of the Cyclades (Modern Greek: Kykládes) group, southeastern Greece, in the Aegean Sea, sometimes included in the Southern Sporades group. It constitutes a dímos (municipality) within the South Aegean (Nótio Aigaío) periféreia (region).

Geologically, Thera is the remaining eastern half of an exploded volcano. Its bow-shaped rim and the remnant isles of Thirasía and Aspronísi form an open lagoon that measures 37 miles (60 km) in circumference. In the centre of the lagoon are two active volcanic islets, Néa Kaméni (“New Burnt Island”) and Palaía Kaméni (“Old Burnt Island”). Thera proper consists largely of lava and pumice, the latter of which is the island’s main export. Red-wine grapes are also grown. The lagoon is rimmed by red-, white-, and black-striped volcanic cliffs rising to almost 1,000 feet (300 metres). The summit of Thera is the 1,857-foot (566-metre) limestone Mount Profítis Ilías in the southeast. The chief town, Thíra (locally called Firá), was badly damaged by an earthquake in 1956. Other settlements include Emboríon and Pírgos to the south and the port of Oía at the north entrance to the lagoon, which was destroyed by the 1956 earthquake.