Valentinian III, Roman emperor from 425 to 455. He was made emperor by Theodosius II, but he never really exercised the imperial power, leaving it in the hands of his mother, Placidia, until her death in 450, and then in the hands of the eunuch Heraclius. Although the barbarians who were constantly harassing the Empire were repeatedly defeated by Aetius, Spain, Africa, Gaul and other provinces were lost to Rome, and it became evident that the Empire was growing steadily weaker. Valentinian was finally assassinated.