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. ^[1] He died of natural causes.

John Cassian wrote the De institutis coenobiorum at the request of Castor.^[2]

His feast day is September 2. His relics are still preserved in the cathedral of Apt, of which he is one of the patrons.