Diemelsee is a municipality in Waldeck-Frankenberg in northwest Hesse, Germany, and is part of the low mountain region called Upland.

The community of Diemelsee came into being as part of municipal reform in 1971 through the voluntary merger of the following 13 centres: The community's administrative seat is in Adorf, which with a population of 1750 is also the biggest of the 13 centres.

The council's 31 seats are apportioned in accordance with municipal elections last held on 26 March 2006.

Diemelsee's civic coat of arms might heraldically be described thus: Party per bend wavy, above, in Or an eight-pointed star sable, below, in azure a five-petalled rose argent with centre Or and sepals vert.

In Adorf is a visitors' mine, the Grube Christiane.