Guilhabert de Castres (about 1165 - 1240) was a prominent Cathar theologian. Born in Castres, he became a Perfect (Parfaits) and, between 1223 and 1226, Bishop of Toulouse in the Cathar Church. In the theological debates in the early 13th century between the Cathars and the Papal representatives, he presented the Cathar arguments, thus at the Debate at Montreal in 1206 and at the last Debate at Pamiers where he encountered Saint Dominic in 1207, prior to the Albigensian crusades.
In 1193, Guilhabert settled in Fanjeaux where he attended to a group of Perfects and believers until Simon de Montfort took the town during the Albigensian Crusade in 1209. Guilhabert withdrew to the Chateau de Montsegur and ministered from its safety. In 1222, Guilhabert escaped from Castelnaudary that was besieged by Amaury de Montford. Five years later, during a respite in the war, he presided over the Cathar synod at the Château de Pieusse where about a hundred Perfects had assembled. During the meeting, it was decided to create the bishopric of Razes. From 1229 to 1232 he lived at the Château du Bézu. Constantly under the threat of persecution, he conducted his pastoral visits to the towns and chateaux of the Languedoc throughout the war.
In 1232, he convinced the lord of the Chateau de Montsegur, Raymond de Péreille (Ramond de Perella), to allow him to set up the "house and head" (domicilium et caput) of the Cathar church at the stronghold, and subsequently the site attracted Cathar refugees and faidits. Guilhabert died in 1240, and was succeeded by Bartrand Marti as bishop.