Grimselsee, reservoir near Grimsel Pass in Guttannen, canton of Bern, Switzerland

Grimselsee is a reservoir in the Aare headwaters above Guttannen, Canton of Bern, held by two dams: the 42-metre Seeuferegg gravity dam and the 114-metre Spitallamm arch-gravity dam, both completed in 1932. The Grimsel Pass road runs along a short stretch of the lake, and the Grimselhospiz stands between the two dam walls. At its upper end the lake meets the Unteraargletscher, just after the Oberaarbach flows in from the left.

The Grimselsee was formed by two dams finished in 1932: the 42-metre Seeuferegg gravity dam and the 114-metre Spitallamm arch-gravity dam, built for the hydroelectric plants of Kraftwerke Oberhasli AG. The Spitallamm dam developed a crack between its air and water faces from the 1960s, so in 2019 work began on a new double-curved arch dam beside it; the old wall is to remain standing in the lake, also for heritage protection.

The lake sits at a high elevation requiring some physical fitness to reach, and weather conditions can change quickly. Visitors should prepare for changeable weather and bring appropriate gear, especially during cooler months.

The water often appears turquoise or milky due to mineral content and glacial fed streams, a visual quality that sets it apart from other Alpine lakes. This coloring shifts noticeably depending on the season and light conditions.