Koos is the largest of several small islands in the Bay of Greifswald, Mecklenburg-Vorpommern, Germany. It has an area of 772 hectares and a maximum elevation of just above three meters. The island is a largely uninhabited natural reserve with restricted access. It is separated from the mainland by Kooser Bucht and Kooser See, two bays connected by a tiny strait, Beek. In 1241, Barnuta, prince of Rügen, granted Koos (then "Chosten") to Eldena abbey (then "Hilda"), later it became a possession of the Hanseatic town of Greifswald, located a few kilometers southward. A medieval burgh has been suggested on the isle, but not verified. A 17th-century Dutch settlement had disappeared in the 18th century. Koos is administered by the nearby town of Greifswald.