Reginald was the son of William II, Duke of Jülich and Maria of Guelders.Walther Möller, Stammtafeln westdeutscher Adelsgeschlechter im Mittelalter (Darmstadt, 1922, reprint Verlag Degener & Co., 1995), Vol. 1, p. 14. He became duke upon his brother William's death in 1402 without issue.
