Pierre de Castelnau (died 15 January 1208), French ecclesiastic, was born in the diocese of Montpellier.

In 1199 he was archdeacon of Maguelonne, and was appointed by Pope Innocent III as one of the legates for the suppression of the Cathar heresy in Languedoc.

In 1202, when a monk in the Cistercian abbey of Fontfroide, Narbonne, he was designated to similar work, first in Toulouse, and afterwards at Viviers and Montpellier.

In 1207 he was in the Rhone valley and in Provence, where he became involved in the strife between the count of Baux and Raymond, count of Toulouse. Castelnau was assassinated on 15 January 1208, quite possibly by an agent of Raymond. He was beatified in the year of his death by Pope Innocent III, who believed Raymond responsible.