Flavius Julius Constans was Roman emperor from 337 to 350. He held the imperial rank of caesar from 333, and was the youngest son of Constantine the Great.
After his father's death, he was made augustus alongside his brothers in September 337. Constans was given the administration of the praetorian prefectures of Italy, Illyricum, and Africa. He defeated the Sarmatians in a campaign shortly afterwards. Quarrels over the sharing of power led to a civil war with his eldest brother and co-emperor Constantine II, who invaded Italy in 340 and was killed in battle with Constans's forces near Aquileia. Constans gained from him the praetorian prefecture of Gaul. Thereafter there were tensions with his remaining brother and co-augustus Constantius II, including over the exiled bishop Athanasius of Alexandria. In the following years he campaigned against the Franks and in 343 visited Roman Britain.
In January 350, Magnentius the commander of the Jovians and Herculians, a corps in the Roman army, was acclaimed augustus at Augustodunum with the support of Marcellinus, the comes rei privatae. Magnentius overthrew and killed Constans.