Flavius Valerius Severus was a soldier from the Illyrian provinces before his associate Galerius appointed him Caesar in 305. On the death of Constantine I in 306, he was promoted Augustus. When Maxentius, the son of the retired emperor Maximian, revolted at Rome, Galerius sent him to suppress the rebellion, but the walls of Rome and the desertion of his own men forced him to retreat to Ravenna, where he surrendered to Maximian.

When Galerius himself invaded Italy in 307 to suppress both Maxentius and his father, Maxentius treacherously had him killed at Tres Tabernae.