The Diemelsee or Diemel Reservoir (German: Diemelstausee) is a reservoir with a surface area of 1.65 km² and about capacity of 19.9 million m³ 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) 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.