Flavius Valerius Severus (d. 16 September, 307) was a Western Roman Emperor.

He was a soldier from the Illyrian provinces before his associate Galerius appointed him Caesar (vice emperor) of the Western Roman Empire in 305. He thus served as a junior partner to Constantius Chlorus, Augustus of the Western Roman Empire.

On the death of Constantius Chlorus in 306, Severus was promoted to 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 Severus killed at Tres Tabernae.