The Grimselsee or Lake Grimsel is an artificial lake near the Grimsel Pass in the Canton of Berne, Switzerland. With a volume of 95 mio m³, it is larger than other hydroelectric reservoirs in the region: Oberaarsee, Räterichsbodensee and Gelmersee. The dam was completed in 1932 and is operated by Kraftwerke Oberhasli AG (KWO). It is located in the municipality of Guttannen.