The Sihlsee (in English sometimes called Lake Sihl) is an artificial lake in the Swiss canton of Schwyz, near the town of Einsiedeln. The lake was created by damming the river Sihl and flooding a section of the upper Sihl Valley.

The lake feeds the Etzelwerk power station, which is located 5 kilometres (3 mi) to the north-east in Altendorf on the upper section of Lake Zürich (the Obersee) and which supplies electricity to the Swiss Federal Railways (SBB). Its concrete dam is 33 metres (108 ft) high and 124 metres (407 ft) m long. The lake is the largest artificial lake of Switzerland in terms of surface with a maximum length of 8.5 kilometres (5 mi) and maximum width of 2.5 kilometres (2 mi). The maximum depth is 17 metres (56 ft), and the lake has an approximate volume of 96,000,000 m (3.4×10 cu ft).