thumb|250px|right|Location of Koos (red) in former [[Ostvorpommern district, Mecklenburg-Vorpommern]]
thumb|250px|View over the Kooser Wiesen ("Koos meadows", on the mainland) towards Koos (marked by the group of trees in the center). The yellow sign showing an owl indicates the area's status as a protected nature reserve.
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. 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.