Kos map Kos or Cos ( / k ɒ s , k ɔː s / ; Greek : Κως [kos] ) 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, after Rhodes and Karpathos ; it has a population of 37,089 (2021 census), making it the second most populous of the Dodecanese after Rhodes. [ 1 ] The island measures 42.1 by 11.5 kilometres (26 by 7 miles). [ 2 ] 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. [ 3 ] https://en.wikipedia.org/wiki/Kos