Pierre de Castelnau was a French Cistercian monk and papal legate active during the late 12th and early 13th centuries. Born in the diocese of Montpellier, he served as the archdeacon of Maguelonne before being appointed by Pope Innocent III in 1199 to combat the Cathar heresy. After joining the Cistercian order at the Fontfroide abbey, he acted as a chief inquisitor in regions including Languedoc, Viviers, and Montpellier. His life ended abruptly in 1208 following an assassination in the Rhone valley or Provence.