Totesee (Germanized: Totensee) is a small natural lake () at the Grimsel Pass in Switzerland. The lake lies immediately to the south of the natural watershed and cantonal boundary at the pass. It is therefore in the canton of Valais, and it would naturally drain into the river Rhône in the valley below. However a dam has been constructed to enable its use as a reservoir, increasing its size and raising its level by . As part of this work, an aqueduct supplies water to the Grimselsee, which drains into the river Aare and thus forms part of the Rhine catchment.