Aelia Galla Placidia (392 – November 27, 450), daughter of the Roman Emperor Theodosius I, was the Regent for Emperor Valentinian III from 423 until his majority in 437, and a major force in Roman politics for most of her life. She was consort to Ataulf king of the Goths from 414-415 until his death, and Empress consort to Constantius III from 417-422 until his death.

Placidia was the daughter of Roman Emperor Theodosius I and his second wife Galla, who was herself daughter of Emperor Valentinian I and his second wife Justina. Her older brother Gratian died young. Her mother died in childbirth in 394, giving birth to John, who died with their mother. Placidia was a younger, paternal half-sister of Emperors Arcadius and Honorius. Her older half-sister Pulcheria predeceased her parents as mentioned in the writings of Gregory of Nyssa, placing the death of Pulcheria prior to the death of Aelia Flaccilla, first wife of Theodosius I, in 385.

Placidia was granted her own household by her father in the early 390s and was thus financially independent while underage. She was summoned to the court of her father in Mediolanum during 394. She was present at Theodosius' death on January 17, 395. She was granted the title of "Nobilissima Puella" ("Most Noble Girl") during her childhood.

Placidia was the wife of Olybrius, 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 Roman naming conventions for females.

Placidia was the second daughter of Valentinian III and Licinia Eudoxia. Her older sister was Eudocia, who became the wife of Huneric, son of Gaiseric, king of the Vandals. Both were named for their grandmothers: Eudocia for their maternal grandmother Aelia Eudocia, and Placidia for their paternal grandmother 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. The Anicii were 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.