Imperial County in northwest Germany;
- An administrative region (Regierungsbezirk) in Nordrhein-Westfalen, named after the town of Arnsberg, which stands on the River Ruhr, ESE of Dortmund.
1. Conrad, one of the Counts of Werl (which is east of Dortmund), built a castle at Arnsberg c.1060, and from it he and his successors were called Counts of Werl-Arnsberg. The last Count lost half of his lands to the Archbishop of Cologne in 1102; when he died in 1124, the position he had held as lay advocate of the Bishopric of Paderborn lapsed, increasing the temporal power of the Bishop, whilst the County passed by an heiress to the family of Cujik, who held it until 1368, when the childless last Count sold it to the Archbishop of Cologne. The Archbishops had been Dukes of Westphalia since the fall of Henry the Lion in 1180, and the lands they had acquired from the Counts of Arnsberg became known as the Duchy ofWestphalia. On the secularisation of the ecclesiastical principalities in 1802-3, the Grand Duke of Hesse was given the Duchy, but in 1815 it passed to Prussia.
2. When the Land of Nordrhein-Westfalen was formed in the British Zone of Occupation in 1946, Arnsberg became its southeastern Regierungsbezirk (administrative region). The town is also the administrative centre of the Hochsauerlandkreis. The largest city in the region is Dortmund, with well over half a million people.