Gelmersee (or Lake Gelmer) is a reservoir in Bernese Oberland, Switzerland. The hydroelectric reservoir was completed in 1932, at about the same time as the Grimselsee reservoir, and both are operated by Kraftwerke Oberhasli. The reservoir's volume is 13 million m³ and its surface area 64.5 ha (159 acres).

The reservoir may be reached by the Gelmerbahn from Handegg, Guttannen at 1,412 m. The funicular's track with a length of 1,028 m has a maximum inclination of 106%.

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

• Lake Gelmer
• Media related to Gelmersee at Wikimedia Commons