Galla Placidia, c.388-450, Roman empress of the West, daughter of Theodosius I. Captured by Alaric I in the course of his Italian campaign, she was held by the Visigoths as a hostage and married (414) Alaric's successor Ataulf. After the murder (415) of Ataulf she was at first ill-treated but was returned in 416 to her brother Honorius. In 417 she married the general Constantius; shortly before his death he was made (421) coemperor as Constantius III. In 423 she quarreled with Honorius and fled to the court of Theodosius II; after the death of Honorius she became regent for her son Valentinian III, whom Theodosius placed on the throne after overthrowing (425) the usurper John. She had great personal influence over her son, but she was forced to leave the government largely in the hands of Aetius.