ruler of the eastern provinces of the Roman Empire, with the title of caesar, from 351 to 354.

Sources dating from this period describe Gallus' reign at Antioch (present-day Antakya, Tur.) as tyrannical. His father, Julius Constantius, was the half brother of Constantine the Great, and Gallus was the elder half brother of the emperor Julian the Apostate and cousin of the emperor Constantius II. Like Julian, with whom he grew up, he was given a strict Christian education. Constantius II proclaimed him caesar at Sirmium (present-day Sremska Mitrovica, Serb.) on March 15, 351, and also arranged Gallus' marriage to his (Constantius') sister, Constantia. But Gallus' strict, isolated upbringing had made him stern and tactless. He instituted a widespread system of espionage among his subjects and executed a number of people—not all unjustly—on suspicion of treason. Meanwhile, he successfully suppressed revolts in Palestine and in Isauria, Galatia (in south-central Anatolia), and held the Persians at bay. His subordinates sent unfavourable and sometimes misleading reports about him to Constantius, prompting the emperor to recall Gallus to Constantinople (present-day Istanbul), to strip him of his powers, and finally to have him executed.