Altastenberg lies in the Hochsauerland district about 2 km northwest of the Kahler Asten (841,9 m) and only a little west of the Bremberg (809 m), about 5 km (as the crow flies) west of Winterberg and between the sources and upper reaches of the river Lenne to the south and the Neger to the northwest. It is located near the crest of the Rothaargebirge mountain range, along which part of the Rhine-Weser watershed runs.