William of Saint-Thierry (French: Guillaume de Saint-Thierry; Latin: Guillelmo S. Theodorici; 1075/80/85–1148) was a twelfth-century French Benedictine abbot of Saint-Thierry, theologian and mystic who became a Cistercian monk and writer.
William was born at Liège (in present-day Belgium) of a noble family between 1075 and 1080 (or 1085), and died at Signy-l'Abbaye in 1148. He probably studied at the cathedral school in Reims, though some have argued it was at Laon, prior to his profession as a Benedictine monk. He became a monk with his brother Simon at the monastery of St. Nicaise, also in Reims, sometime after 1111. From here both eventually became abbots of other Benedictine abbeys: Simon at the abbey of Saint-Nicolas-au-Bois, in the Diocese of Laon, and William at Saint-Thierry, on a hill overlooking Reims, in 1119.
In 1118 William met St. Bernard, abbot of the Cistercian monastery of Clairvaux, where they formed an intimate friendship that lasted for life. His greatest desire was to move to Clairvaux and profess as a Cistercian, but Bernard disapproved of the plan and imposed on him the responsibility of remaining in charge of the abbey at St. Thierry as a Benedictine. Their friendship, however, grew stronger while Bernard lay in the infirmary of Clairvaux convalescing after an illness in 1125.
William was instrumental in the first General Chapter meeting of the Benedictine abbots in the Diocese of Reims, in 1131, and it is possible that he hosted the chapter meeting at Saint-Thierry. After the second General Chapter of the Benedictines, held at Soissons in 1132, where many Cistercian reforms were adopted by the Benedictines, William submitted his Responsio abbatum ("Response of the Abbots") to Cardinal Matthew — papal legate in the diocese and critic of the abbots' reforms — successfully defending their reformation efforts. On account of long infirmities and a lifelong desire for a life of contemplation, William resigned his abbacy in 1135 and entered the newly established Cistercian Signy Abbey, also in the diocese of Reims. He did not venture to retire to Clairvaux lest his friend Bernard refuse to accept his abdication. There he divided his free time between prayer, study, and writing. According to a contemporary, his death occurred in 1148, about the time of the council held at Reims under Pope Eugenius. The necrology of Signy dates it 8 September, a few years prior to his good friend Bernard's death in 1153.