William of Champeaux (c. 1070 – 1121) was a French philosopher and theologian.

He was born at Champeaux near Melun. After studying under Anselm of Laon and Roscellinus, he taught in the school of the cathedral of Notre-Dame, of which he was made canon in 1103. Among his pupils was Pierre Abélard. In 1108 he retired into the abbey of St Victor, where he resumed his lectures. He afterwards became bishop of Châlons-en-Champagne, and took part in the dispute concerning investitures as a supporter of Pope Callixtus II, whom he represented at the conference of Mousson .

His only printed works are a fragment on the Eucharist (inserted by Jean Mabillon in his edition of the works of St Bernard), and the Moralia A brevi ala and De Origine Animae (in E. Martnes Thesaurus novus Anecdotorum, 1717, vol. 5). In the last of these he maintains that children who die unbaptized must be lost, the pure soul being defiled by the grossness of the body, and declares that God's will is not to be questioned. He upholds the theory of Creationism (that a soul is specially created for each human being). Ravaisson-Mollien has discovered a number of fragments by him, among which the most important is the De Essentia Dei et de Substantia Dei; a Liber Sententiarum, consisting of discussions on ethics and Scriptural interpretation, is also ascribed to Champeaux. He is reputed the founder of Realism.