Galla Placidia (388–89/392–93 – 27 November 450), daughter of the Roman emperor Theodosius I, was a mother, tutor, and advisor to emperor Valentinian III, and a major force in Roman politics for most of her life. She was queen consort to Ataulf, king of the Visigoths from 414 until his death in 415, briefly empress consort to Constantius III in 421, and managed the government administration as a regent during the early reign of Valentinian III, until her death.
Placidia was the daughter of Theodosius I and his second wife, Galla, who was herself daughter of Valentinian I and his second wife, Justina. Galla Placidia's date of birth is not recorded, but she must have been born either in the period 388–89 or 392–93. Between these dates, her father was in Italy following his campaign against the usurper Magnus Maximus, while her mother remained in Constantinople.
A surviving letter from Bishop Ambrose of Milan, dated 390, refers to a younger son of Theodosius named Gratianus, who died in infancy; as Gratian must have been born in the period 388–89, it is most probable that Galla Placidia was born during the second period, 392–93. Placidia's mother Galla died some time in 394, perhaps giving birth to a stillborn son.
Placidia was a younger, paternal half-sister of emperors Arcadius and Honorius. Her older half-sister Pulcheria predeceased her parents according to Gregory of Nyssa, placing the death of Pulcheria prior to the death of Aelia Flaccilla, the first wife of Theodosius I, in 385. Coins issued in Placidia's honour in Constantinople after 425 give her name as AELIA PLACIDIA; this may have been intended to integrate Placidia with the eastern dynasty of Theodosius II. There is no evidence that the name Aelia was ever used in the west, or that it formed part of Placidia's official nomenclature.
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 (Milan) during 394, and was present at Theodosius' death on 17 January 395.