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 (Placidia, Aelia Galla). 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 (Aetius, Flavius) became the effective ruler toward the end of this regency. The most important political event of these years was the landing of the Vandals (Vandal) 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.

On October 29, 437, Valentinian married Licinia Eudoxia, the daughter of Theodosius II (Eastern emperor, 408–450) and Eudocia. Little is known of Valentinian in the years after his marriage. He spent his life in the pursuit of pleasure while Aetius controlled the government. In 444 Valentinian, acting in conjunction with Pope Leo I the Great (Leo I, Saint), issued the famous Novel 17, which assigned to the bishop of Rome supremacy over the provincial churches. During the closing years of Valentinian's reign, the Huns (Hun) invaded Gaul (451) and northern Italy (452), but it is not known whether Valentinian personally played any significant part in meeting these crises.

As a result of false information that made him doubt Aetius's loyalty, Valentinian murdered the great patrician with his own hands in the imperial palace at Rome on September 21, 454. The following year, two barbarians, Optila and Thraustila, who had been retainers of Aetius, avenged their master by murdering the emperor in the Campus Martius.