Constantius III was a late Roman Emperor.

He first gained note by his successful campaigns to save the Western Roman Empire, which had been seriously weakened by both barbarian invasions, and the revolt of Constantine III. As a result, first he was made Patrician, then in 417 he was married to the Emperor Honorius's sister, Placidia, and finally was elevated to co-Emperor with Honorius on February 8, 421. He died later that year, on September 2.

Their children included the Emperor Valentinian III.

Constantius's success in rising from head of the dwindling Roman army to Imperial rank obviously influenced the actions of later holders of the patrician office, a list that includes Aetius and Ricimer.