Diemelsee, Reservoir in Waldeck-Frankenberg and Hochsauerlandkreis, Germany

Diemelsee is a reservoir on the Diemel and Itter with a surface of 1.65 km² and about 19.9 million m³ of storage. It lies mostly in the Waldeck-Frankenberg county of North Hesse, while its smaller northern part and the dam belong to Marsberg in the Hochsauerlandkreis. The dam stands about 500 m south of Helminghausen; Heringhausen is the only village on the shore.

Construction work on the dam began in 1912 and lasted until 1923, interrupted by World War I. The facility was meant to regulate floods and provide hydropower, which represented a technical innovation for the region at the time.

The name comes from the Diemel river, which joins the Itter here to form a branched water body with quiet bays. Hikers follow a circular trail along the shore, while sailors and anglers prefer the calmer inlets.

The shore road connects several villages around the lake and offers parking with water access at many points. Those who want to explore the area on foot will find marked trails through the forest and along the bay.

Beavers released from Bavarian stock live at the reservoir's inflows; between 2017 and 2023 five of them fell down the spillway and died while trying to reach the Diemel below the dam. In March 2023 one survived the fall and was taken to the Essenthoer Mühle rescue station. Six beaver pairs were counted at the lake and its feeders in early 2023.