Valentinian III (Latin: Flavius Placidius Valentinianus Augustus; 2 July 419 – 16 March 455) was Western Roman Emperor from 425 to 455. His reign was marked by the ongoing dismemberment of the Western Empire.

Valentinian was born in the western capital of Ravenna, the only son of Galla Placidia and Flavius Constantius. His mother was the younger half-sister of the western emperor Honorius, while his father was at the time a Patrician and the power behind the throne.

Through his mother, Valentinian was a descendant both of Theodosius I, who was his maternal grandfather, and of Valentinian I, who was the father of his maternal grandmother. It was also through his mother's side of the family that he was the nephew of Honorius and first cousin to Theodosius II (the son of Honorius' brother Arcadius), who was eastern emperor for most of Valentinian's life. Valentinian had a full sister, Justa Grata Honoria, who was probably born in 417 or 418 (the history of Paul the Deacon mentions her first when mentioning the children of the marriage, suggesting she was the eldest). His mother had previously been married to Ataulf of the Visigoths, and had borne a son, Theodosius, in Barcelona in 414; but the child had died early in the following year, thus eliminating an opportunity for a Romano-Visigothic line.

When Valentinian was less than two years old, Honorius appointed Constantius co-emperor, a position he would hold until his death seven months later. As a result of all these family ties, Valentinian was the son, grandson, great-grandson, cousin, and nephew (twice over) of Roman Emperors.

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