One of eight glacial lakes of the Šumava and the Bavarian Forest lies on the slope of Plechý (1878 m), the highest mountain of the Šumava. The water sheet of 7,48 hectares with the perimeter of 1242 m stretches out at the territory of the Šumava National Park, 6 km to the west from Nová Pec and at the altitude of 1090 m.
The 540 m long and 150 m wide lake has a maximum depth of 18,3 m. The lake has and interesting feature in form of the front moraine and the lake wall, which is some 220 m high. In its right part there is a granite monument to the memory of the local poet from Horní Planá, Adalbert Stifter. The lake is the only endemic locality of Spring Quillwort and the Heterocope saliens in the Czech Republic. The lake is drained off through the Jezerní stream which flows into the Vltava. Since the interconnection of the Jezerní stream with the Schwarzenberg floating channel in 1781, Plešné Lake operated as a reservoir of water drained as needed for purposes of timber rafting. About 200 m below the lake there is a stone sea of granite boulders extending at the area of 6 hectares.