Agios Efstratios or Saint Eustratius, colloquially Ai Stratis (Greek: Άη Στράτης) is a small Greek island in the northern Aegean Sea about 30 kilometres (19 miles) southwest of Lemnos and 80 kilometres (50 miles) northwest of Lesbos. Together with Lemnos and nearby islets it forms the regional unit of Lemnos, part of the Greek archipelagic region of the North Aegean.
Agios Efstratios is quiet, isolated and dry, with a population of 270 people. The climate is arid, with little rainfall during the winter months and long, hot summers. The landscape is mostly rocky, with scarce and low vegetation.
Crops are insignificant; the surrounding sea, however, is rich in fish which are fished by local fishermen.
There are numerous beaches on the island such as Agios Antonios, Lemonies, Avlakia and others, most of which are reached by caique.