Castor of Apt (died c. 420) was a bishop of Apt, in Gaul. He was born in Nîmes and may have been the brother of Leontius of Fréjus. Castor was a lawyer and married to a wealthy widow. He lived in Marseilles. His wife, however, allowed him to enter the religious life; she herself entered a nunnery. Castor founded the monastery of Manauque (Monanque) in Provence which followed the monastic rule of John Cassian. He was subsequently made bishop of Apt. He died of natural causes. John Cassian wrote the De institutis coenobiorum at the request of Castor. His feast day is September 2. His relics are still preserved in the cathedral of Apt, of which he is one of the patrons.