Valentinian III (Latin: Flavius Placidus Valentinianus; 2 July 419 – 16 March 455) was Roman emperor in the West from 425 to 455. Made emperor in childhood, his reign over the Roman Empire was one of the longest, but was dominated by powerful generals vying for power amid civil wars and the invasions of Late Antiquity's Migration Period, including the campaigns of Attila the Hun.

He was the son of Galla Placidia and Constantius III, and as the great-grandson of Valentinian I (r. 364–375) he was the last emperor of the Valentinianic dynasty. As a grandson of Theodosius I (r. 379–395), Valentinian was also a member of the Theodosian dynasty, to which his wife, Licinia Eudoxia, also belonged. A year before assuming the rank of augustus, Valentinian was given the imperial rank of caesar by his half-cousin and co-emperor Theodosius II (r. 402–450). The augusta Galla Placidia had great influence during her son's rule. During his early reign Aetius, Felix, and the comes africae, Bonifacius all competed for power within the western empire. Eventually Aetius would defeat Felix and Bonifacius. Aetius would go on to campaign against the many Germanic tribes invading the empire.

During Valentinian's reign the Huns invaded the Roman Empire. Eventually Aetius would defeat the Huns at the Battle of the Catalaunian Plains. Once the Huns returned, Pope Leo I and two other senators convinced Attila to leave. Valentinian himself killed Aetius, and in response Aetius's bodyguards assassinated Valentinian. Valentinian's reign was marked by the ongoing collapse of the western empire.