William, Count of Arnsberg

William of Arnsberg (c. 1277–1338) succeeded his father Louis as count of Arnsberg in 1313. A settlement mediated by his uncle Gerhard of Jülich confirmed him as sole heir, and he then held a court registering 261 fiefs. He backed Louis of Bavaria for the German crown, gaining Soest's advocacy and ducal rights in his county.

In advanced age he joined John of Luxembourg's campaign with the Teutonic Order against the Lithuanians, founding Arensburg on Ösel in 1337. He died in 1338.