Flavius Claudius Constantius Gallus (ca. 325/326–354), commonly known as Constantius Gallus, was a member of the Constantinian dynasty and Caesar of the Roman Empire (351–354). Gallus was consul three years, from 352 to 354.

Gallus was a son of Julius Constantius by his first wife Galla. Gallus' paternal grandparents were the Western Roman Emperor Constantius Chlorus and his second wife Flavia Maximiana Theodora. Julius Constantius was a paternal half-brother of the Roman Emperor Constantine I, which, in turn, meant Gallus was a half-first cousin of Constantine's sons, Constantine II, Constantius II and Constans. At some point prior to 331/2, Gallus' mother, Galla, seems to have died, as at that stage, Gallus' father was married to one Basilina and had had a son by that marriage.

Gallus had three siblings: an elder sister, of unknown name, who was the first wife of Constantius II, an elder brother, also of unknown name, who died in the purges after the death of Constantine I, and a younger half-brother by his father's second marriage, named Flavius Claudius Iulianus, commonly known as Julian.

Gallus was said by Ammianus Marcellinus to have been very good looking, making particular note of his soft blond hair.

Gallus was born in Massa Veternensis, Italia, after his father had been recalled from exile. Gallus' father and his elder brother were amongst those killed during the purges that occurred in the imperial family after the death of Constantine I in 337. Gallus himself was one of the only imperial males, outside of the three sons of Constantine I and Fausta, who were not killed; the others being Gallus' younger half-brother, Julian, and their cousin, Nepotian, each of whom was very young at the time.

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