Aelia Galla Placidia (388 – 27 November 450), daughter of the Roman emperor Theodosius I, was regent to Valentinian III from 423 until his majority in 437, 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, and briefly empress consort to Constantius III in 421.

Placidia was the daughter of Theodosius I and his second wife, Galla, who was herself daughter of 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 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.

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 spent most of her early years in the household of Stilicho the Vandal and his wife, Serena. She is presumed to have learned weaving and embroidery. She might have also been given a classical education, though no details are known. Serena was a first cousin of Arcadius, Honorius and Placidia. The poem "In Praise of Serena" by Claudian and the Historia Nova by Zosimus clarify that Serena's father was an elder Honorius, a brother to Theodosius I. According to "De Consulatu Stilichonis" by Claudian, Placidia was betrothed to Eucherius, only known son of Stilicho and Serena. Her scheduled marriage is mentioned in the text as the third union between Stilicho's family and the Theodosian dynasty, following those of Stilicho to Serena and Maria, their daughter, to Honorius.

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