Agios Efstratios, also known as Saint Eustratius or colloquially Ai Stratis, is a small Greek island located in the northern Aegean Sea. Historically, the island was referred to by the ancient name Halonnesus or Halonnesos.
The island is situated approximately 30 kilometres southwest of Lemnos and 80 kilometres northwest of Lesbos. It covers a total area of 43.325 km2 and belongs to the municipality within the regional unit of Lemnos, which is part of the Greek archipelagic region of the North Aegean.