Arnsberg, a town of Prussia, capital of a district of the same name in the province of Westphalia, situated on a hill partly surrounded by the Ruhr, 44 m. S. S. E. of Minister; pop. in 1871, 4,734. The new portion of the town dates from the early part of this century. In the vicinity are the ruins of an old castle where the famous Vehmic court used to be held. The once important county of Arnsberg came in 1368 into the possession of the electorate of Cologne, and the town was subsequently for some time capital of the duchy of Westphalia, of which Arnsberg is the most populous and prosperous district.