Klöntalersee is a natural lake in the Klöntal valley in the Canton of Glarus , Switzerland. It was formed by a rockslide and has served as a reservoir for hydroelectric power generation since 1908. The lake is impounded by the Rhodannenberg earthfill dam, which supplies water to a hydroelectric power station near Netstal .

Located at an elevation of 848 metres above sea level, the lake covers an area of approximately 3.3 square kilometres. It is considered the oldest major reservoir in Switzerland and is known for its frequently mirror-like surface that reflects the surrounding mountains. The primary inflow is the Klön stream, and the outflow is the Löntsch , which flows into the Linth near Netstal.