Kos or Cos is a Greek island, which is part of the Dodecanese island chain in the southeastern Aegean Sea. Kos is the third largest island of the Dodecanese by area, after Rhodes and Karpathos; it has a population of 36,986 (2021 census), making it the second most populous of the Dodecanese, after Rhodes. The island measures 42.1 x. Administratively, Kos constitutes a municipality within the Kos regional unit, which is part of the South Aegean region. The principal town of the island and seat of the municipality is the town of Kos.