The Diemelsee or Diemel Reservoir (German: Diemelstausee) is a reservoir with a surface area of 1.65 km²[1] and about capacity of 19.9 million m³[1] on the River Diemel in the counties of Waldeck-Frankenberg in North Hesse, and Hochsauerlandkreis, Westphalia, Germany.

It is part of the Diemeltalsperre hydropower system (DiT)[3] comprising the Diemel Dam, the equalizing basin, the power plant and the reservoir itself, owned by the Federal Waterway and Navigation Authority and managed by its Hann.

The Diemel Reservoir, along with the Edersee is part of the water regulation structure in the catchment area of the River Weser.

Its dam stands about 500 metres south of the village of Helminghausen (southwest of Marsberg).

The Diemelsee lies in a forested Central Upland countryside, whose highest point in the vicinity of the reservoir is the Köpfchen (ca.