Castinus of Byzantium was a Roman senator who became a prominent early Christian leader. After converting to the faith through baptism by Cyrillianus, the bishop of Argyropolis, he dedicated his personal wealth to supporting the impoverished. He served as the bishop of Byzantium from 230 until his death in 237. Although some historical accounts, such as those by Frederick George Holweck, question the existence of bishops in Byzantium during this century, he remains recognized in various traditions.