The Walensee (German pronunciation: [ˈvaːln̩ˌzeː] ), also known as Lake Walen and Lake Walenstadt (after Walenstadt), is one of the larger lakes in Switzerland . Located in the east of the country, about two thirds of its area are in the Canton of St. Gallen and about one third in the Canton of Glarus .

Its name means 'Lake of the Walhaz' (German: See der Welschen), since in the early Middle Ages Walensee formed the linguistic border between the Alemanni, who settled in the west, and the Romansh people, the Walhaz (Welschen), in the east.

The lake lies in a valley between the Appenzell Alps to the north and the Glarus Alps to the south. It has a long east-west extension but is relatively narrow in north-south direction, with a surface area of 24 km2 (9.3 sq mi).