Agios Efstratios or Saint Eustratius, colloquially Ai Stratis, anciently Halonnesus or Halonnesos, is a small Greek island in the northern Aegean Sea about 30 kilometres southwest of Lemnos and 80 kilometres northwest of Lesbos. The municipality has an area of 43.325 km2. Together with Lemnos and nearby islets it forms the regional unit of Lemnos, part of the Greek archipelagic region of the North Aegean.