(Καισάρειος), a physician who is however better known as having been the brother of St. Gregory Theologus.

He was born of Christian parents, his father (whose name was Gregory) being bishop of Nazianzus. He was carefully and religiously educated, and studied at Alexandria, where he made great progress in geometry, astronomy, arithmetic, and medicine. He afterwards embraced the medical profession, and settled at Constantinople, where he enjoyed a great reputation, and became the friend and physician of the emperor Constantius, A. D. 337-360. Upon the accession of Julian, Caesarius was tempted by the emperor to apostatize to paganism; but he refused, and chose rather to leave the court and return to his native country. After the death of Julian, he was recalled to court, and held in high esteem by the emperors Jovian, Valens, and Valentinian, by one of whom he was appointed quaestor of Bithynia. At the time of the earthquake at Nicaea, he was preserved in a very remarkable manner, upon which his brother St. Gregory took occasion to write a letter (which is still extant, Ep. 20, vol. ii. p. 19, ed. Paris, 1840), urging upon him the duty of abandoning all worldly cares, and giving himself up entirely to the service of God. This he had long wished to do, but was now prevented from putting his design into execution by his death, which took place A. D. 369, shortly after his baptism. His brother pronounced a funeral oration on the occasion, which is still extant (Orat. 7, vol. i. p. 198), and from which the preceding particulars of his life are taken; and also wrote several short poems, or epitaphs, lamenting his death. (Opera, vol. ii. p. 1110, &c.)

The memory of St. Caesarius is celebrated in the Romish Church on Feb. 25.