Castor of Apt was a 4th-century bishop in Gaul who was born in Nîmes. After studying in Arles, he practiced law and successfully defended a widow and her daughter in court. Following the death of his mother-in-law, he and his wife transitioned into religious life. He established the monastery of Saint-Faustin in Provence, which adhered to the rules of John Cassian. He later served as a bishop and was involved in a synod called by Pope Boniface I to address ecclesiastical matters.