Großer Plöner See, Natural lake in Holstein Switzerland, Germany

Großer Plöner See is the largest and deepest lake in Schleswig-Holstein, covering about 28 km² and reaching 56 m in its eastern Plön Basin, while the western Ascheberg Basin is shallower at 30 m. Two glacial basins form the lake, joined by a shallow channel; more than 20 islands lie within it, and the 2 km Prinzeninsel peninsula reaches north. Plön Castle stands on a hill above the north shore.

The lake area shows traces of human settlement from the Middle Stone Age through medieval times, with Slavic settlers leaving their mark on the landscape. The present-day villages and settlements around the water developed gradually over centuries.

The Slavic name Plune, meaning "ice-free water", was recorded for the fort on the Olsborg island south of Plön, destroyed in 1139 by Count Adolf II of Schauenburg; the town's name comes from it. Today the lake is fished by professional fishermen and pleasure boats cross it in summer.

The lake has 15 bathing sites, and campsites sit directly on the shore at Bosau and Ruhleben. Boat rentals are in Plön, and slipways along the shore let visitors launch their own craft. A single guest fishing permit covers the whole lake, and night fishing is allowed in summer for an extra fee.

The Schwentine River flows through the lake, creating a natural waterway that connects the inland water directly to the Kiel Fjord and the Baltic Sea. This connection historically made the lake important for navigation between the interior region and the coast.