Kos or Cos is a Greek island, 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 33,388 (2011 census), making it the second most populous of the Dodecanese, after Rhodes. The island measures 40 by 8 kilometres (25 by 5 miles). 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 Kos Town.

Tourism is the main industry in Kos, the island's beaches being the primary attraction. The main port and population centre on the island, Kos town, is also the tourist and cultural centre, with whitewashed buildings including many hotels, restaurants and a number of nightclubs forming the Kos town "barstreet". The seaside village of Kardamena is a popular resort for young holidaymakers (primarily from the United Kingdom and Scandinavia) and has a large number of bars and nightclubs.

Farming is the second principal occupation, with the main crops being grapes, almonds, figs, olives, and tomatoes, along with wheat and corn. Cos lettuce (romaine lettuce) is named after the island, from where it is said to have originated.