Lake Thun (German: Thunersee) is an Alpine lake in the Bernese Oberland in Switzerland. 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 Aare (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 glacial period. Originally and up to the tenth century, present Lake Thun and Lake Brienz were one long lake, called Wendelsee ("Lake Wendel"). The culminating point of the lake's drainage basin is the Finsteraarhorn at 4,274 metres above sea level. Since 1835 passenger ships have operated on the lake. There are ten passenger ships in total, operated by the local railway company BLS AG. In order to enable these ships to serve the towns of Interlaken and Thun, the Interlaken ship canal and Thun ship canal connect the lake to Interlaken West railway station and Thun railway station respectively.
Fishing is important enough to keep a handful of professional fishers employed. In 2001 the total catch was 53,000 kg. Following World War II and up until 1964, the Swiss Government disposed of unused munitions into Lake Thun. The quantity of munitions dumped is reported to be from 3,000 to more than 9,020 tons.