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.

• List of lakes of Switzerland
• List of mountain lakes of Switzerland

• Media related to Grimselsee at Wikimedia Commons
• KWO: Lake Grimsel