The Grimselsee or Lake Grimsel is an artificial lake near the Grimsel Pass in the Canton of Bern, Switzerland.

With a volume of 95 mio m3 (20.9 billion imperial gallons, 20.1 billion US gallons), it is larger than other hydroelectric reservoirs in the Aare headwaters 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.

This article related to a lake in the canton of Bern is a stub.