The Schwentine is a river in the North German state of Schleswig-Holstein. It is approximately 62 kilometres (39 mi) long and rises on the hill of Bungsberg, the highest point in the state, near the village of Kasseedorf in Ostholstein. It then runs from its source to Kiel where it flows into the Kiel Fjord, a bay of the Baltic Sea. It passes through several lakes, including the Großer Plöner See, the largest lake in Schleswig-Holstein, as well as the towns Eutin, Malente, Plön, Preetz and Kiel.

The Großer Plöner See or Lake Plön is the largest lake (30 km2) in Schleswig-Holstein, Germany. It is located near the town of Plön. Its main tributary, as well as its main outflow, is the River Schwentine.