Roman empress, the daughter of the emperor Theodosius I (ruled 379–395), sister of the Western emperor Flavius Honorius (ruled 393–423), wife of the Western emperor Constantius III (ruled 421), and mother of the Western emperor Valentinian III (ruled 425–455).

Captured in Rome when the city fell to the Goths in 410, she was carried off to Gaul and married (414) to the Visigothic chieftain Ataulphus, who was assassinated in 415. In 416 Galla Placidia was restored to the Romans, and the following year she was married to Constantius. She adorned Ravenna with a number of churches; the small chapel usually—though wrongly—known as the Mausoleum of Galla Placidia contains some of the finest examples of early Byzantine mosaics.