Chartreuse de Saïx, Carthusian monastery in Castres, France

Chartreuse de Saïx is a former Carthusian monastery on the left bank of the Agout at Boussac, in Castres. Of the monastic buildings, razed after the Revolution, only the enclosing wall, a basin and a large Louis XIV-style stone gateway survive; the gateway has a moulded arch, a weathered frieze and a plaque reading "Chartreuse de Saïx". Two small square towers with pyramidal roofs punctuate the long wall.

Raymond Saysse, a Castres merchant, and his wife Centulie de Brettes planned the foundation from 1359, but it was executed only after his death on 21 July 1361, when his will gave the Bossard farmstead by the Agout to the new house. On 7 December 1423 it absorbed the charterhouse of La Loubatière, confirmed by Pope Martin V in 1427. Sacked by the Protestant captain Guilhot de Ferrières, the monks fled to Toulouse in 1569; they returned in 1647 and the monastery was entirely rebuilt by 1674. Dissolved in December 1791, it was used as a prison during the Terror, then sold and razed.

The name Bellevue (originally Belvezer or Belvèse) was kept even after the community moved to Toulouse in 1569, where the monks settled in the Saint-Pierre priory. The monks of Castres long continued to style themselves religious of La Loubatière after absorbing that house in 1423. The site later gave its name to the Chartreuse de Castres industrial zone.

Only the enclosing wall, a large Louis XIV-style stone gateway, a basin and a fishpond in the bend of the Agout survive; the monastic buildings were razed after the Revolution. The gateway stands at the end of a dirt path, and the walled grounds with their two small square towers are best followed on foot. The site is listed as a monument historique since 24 January 1978.

The monastery came into being through the will of a wealthy merchant rather than royal or church patronage like most Carthusian houses. This origin from private wealth made it an unusual founding story in the region.