Valentinian III, Latin in full Flavius Placidius Valentinianus, (born July 2, 419, Ravenna [Italy]—died March 16, 455, Rome), Roman emperor from 425 to 455. At no time in his long reign were the affairs of state personally managed by Valentinian. He was the son of the patrician Flavius Constantius (who ruled as Constantius III in 421) and Galla Placidia. When his uncle, the emperor Honorius, died in 423, the usurper John ruled for two years before he was deposed. Then Placidia controlled the West in her young son’s name until 437, although the powerful patrician Flavius Aetius became the effective ruler toward the end of this regency. The most important political event of these years was the landing of the Vandals in Africa in 429; 10 years later they threw off the overlordship of Valentinian’s government. Valentinian was utterly unable to stop their attacks on Italy.