- died 15 January 1208), French ecclesiastic, made papal legate in 1199 to address the Cathar heresy, he was subsequently murdered in 1208.

Following his death Pope Innocent III beatified him by papal order, excommunicated Count Raymond VI of Toulouse, and declared the Albigensian crusade.

He became archdeacon of Maguelonne,[1] and in 1199 was appointed by Pope Innocent III as one of the papal legates for the suppression of the Cathar heresy in Languedoc.

[2] In 1202, he made profession as a Cistercian monk at the abbey of Fontfroide,[2] Narbonne, and by 1203 was confirmed as papal legate and chief inquisitor, first in Languedoc,[3] and afterwards at Viviers and Montpellier.

[9] The relics of Pierre de Castelnau are interred in the church of the ancient Abbey of St-Gilles.