Aelia Flavia Flaccilla (31 March 356 – 386), was a Roman empress and first wife of the Roman Emperor Theodosius I. She was of Hispanian Roman descent. During her marriage to Theodosius, she gave birth to two sons — future Emperors Arcadius and Honorius — and a daughter, Aelia Pulcheria. She was titled Augusta, as her coinage shows.

According to Laus Serenae ("In Praise of Serena"), a poem by Claudian, both Serena and Flaccilla were from Hispania.

A passage of Themistius (Oratio XVI, De Saturnino) has been interpreted as identifying Flavius Claudius Antonius, Praetorian prefect of Gaul from 376 to 377 and Roman consul in 382, to be her father. However the relation is considered doubtful. In 1967, John Robert Martindale, later one of several article writers in the Prosopography of the Later Roman Empire, suggested that the passage actually identifies Antonius as the brother-in-law of Theodosius. However the passage is vague enough to allow Flavius Afranius Syagrius, co-consul of Antonius in 382, to be the brother-in-law in question.

The only kin clearly identified in primary sources was her nephew Nebridius, son of an unnamed sister. He married Salvina, a daughter of Gildo. Their marriage was mentioned by Jerome in his correspondence with Salvina. They had a son and a daughter.

In about 375–376, Flaccilla married Theodosius I, a son of Count Theodosius. At the time Theodosius had fallen out of favor with Valentinian I and had withdrawn to civilian life in Cauca, Gallaecia.

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