Lake Thun (German: Thunersee) is an Alpine lake in the Bernese Oberland. It took its name from the city of Thun, on its northern shore.
Lake Thun's approximately 2,500 km² large catchment area frequently causes local flooding after heavy rainfalls. This occurs because the river Aar (German: Aare), which drains Lake Thun, has only limited capacity to handle the excess runoff.
The lake is fed by water from Lake Brienz to the south east, which lies 6 metres higher than Lake Thun, and various streams in the Oberland, including the Kander (only since 1714). Lake Thun was created after the last ice age and was originally part of Lake Brienz. The historic combined lake is called Wendelsee by geologists and historians.
Fishing is important enough to keep a handful of professional fishers employed. In 2001 the total catch was 53,000 kg. Since 1835 passenger ships have operated on the lake. There are ten passenger ships in total, operated by the local railway company BLS Lötschbergbahn.