The Aar Dam and its reservoir, the Aartalsee, lie in the upper Aar valley in the German state of Hesse. Both are in the municipalities of Bischoffen and Hohenahr in the county of Lahn-Dill-Kreis and about 15 km northwest of the town of Gießen in Hesse. On the lakeshores lie the villages of Ahrdt and Mudersbach, both part of the parish of Hohenahr.

 

Since 1991 the dam has impounded the River Aar, a left-hand tributary of the Dill in the Gladenbach Uplands. The reservoir is also fed by the Stadterbach, Wilsbach (into the forebay), Weidbach and Meerbach (into the main reservoir).