William of Hirsau (or Wilhelm von Hirschau) (c. 1030 – 5 July 1091) was a Benedictine abbot and monastic reformer. He was abbot of Hirsau Abbey, for whom he created the Constitutiones Hirsaugienses, based on the uses of Cluny, and was the father of the Hirsau Reforms, which influenced many Benedictine monasteries in Germany. He supported the papacy in the Investiture Controversy. In the Roman Catholic Church, he is a Blessed, the second of three steps toward recognition as a saint.