Placidia IPA: [pɫa'kɪdɪa] was the wife of Olybrius, Unrecognized Western Roman Emperor. Her full name is uncertain. The Chronicle of the Roman Emperors: The reign by reign record of the rulers of Imperial Rome (1995) by Chris Scarre gives her name as Galla Placidia Valentiniana or Galla Placidia the Younger, based on naming conventions for women in ancient Rome.

Placidia was the second daughter of Valentinian III and Licinia Eudoxia, younger sister of Eudocia, who became the wife of Huneric, son of Gaiseric, king of the Vandals. Both were named for their grandmothers: Eudocia for the maternal, Aelia Eudocia, and Placidia for the paternal, Galla Placidia. Placidia is estimated to have been born between 439 and 443.

In 454 or 455, Placidia married Anicius Olybrius, a member of the Anicii family., a prominent family with known members active in both Italia and Gaul. The exact relation of Olybrius to other members of the family is not known as his parents are not named in primary sources. Several theories exist as to their identity.

Originally Emperor Valentinian intended to marry Placidia to a young man named Majorian (the future emperor), whom Oost describes as having "distinguished himself in a subaltern capacity fighting in Gaul against the Franks under Aëtius' own command." Doing so, according to Roman customs, would instantly link Majorian to the Imperial family and put him in line to succeed Valentinian. Once Flavius Aetius learned of this plan, he rusticated Majorian to his estates at some date before 451, and he was recalled to Rome only after Aetius' death. Aetius also attempted to consolidate his position "by compelling the Emperor to swear to friendship with him and to agree to betroth Placidia to his own younger son Gaudentius."

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