The Karl Ernst Osthaus-Museum is an art museum in Hagen, North Rhine-Westphalia, Germany. The center of the museum is a building whose interior was designed by Henry van de Velde to house Karl Ernst Osthaus' art collection, open to the public as the Folkwang Museum. When Osthaus' heirs sold his art collection to the city of Essen, the city of Hagen gained possession of the empty museum building.