William I, Duke of Jülich

William I (c. 1299–1361) ruled Jülich as count from 1328, margrave from 1336 and, from 1356, as its first duke. Eldest son of Count Gerhard V, he married Joanna of Hainaut in 1324, making him brother-in-law to Edward III of England. In 1337 he helped form the German-English alliance that opened the Hundred Years' War, later backing Charles IV, who raised him to duke.

His sons imprisoned him during a revolt of Jülich knights in 1349, but he was freed in 1351. He died in 1361 and was followed by his second son, William II.