Lake Thun (German: Thunersee, Swiss Standard German pronunciation: [ˈtuːnərˌzeː]) is an Alpine lake in the Bernese Oberland in Switzerland named after the city of Thun, on its northern shore. At 48.3 km2 (18.6 sq mi) in surface area, it is the largest Swiss lake entirely within a single canton.

The lake was created after the last glacial period. After the 10th century, it split from Lake Brienz , before which the two lakes were combined, as Wendelsee ("Lake Wendel"). The culminating point of the lake's drainage basin is the Finsteraarhorn at 4,274 metres (14,022 ft) above sea level.

Lake Thun's approximate 2,500 square kilometres (970 sq mi) catchment area frequently causes local flooding after heavy rainfalls.