 Diemelsee lies in the River Diemel in the Diemelsee Nature Park just upstream from and southwest of the Diemelsee reservoir.The town of Korbach is found about 10 km to the southeast, and the town of Paderborn 39 km to the north.Diemelsee borders in the north on the town of Marsberg (Hochsauerlandkreis in North Rhine-Westphalia), in the northeast on the town of Bad Arolsen, in the east on the community of Twistetal, in the south on the town of Korbach, in the southwest on the community of Willingen (all in Waldeck-Frankenberg), and in the west on the town of Brilon (Hochsauerlandkreis).