Flavius Valerius Severus, of Illyrian stock, was a friend of emperor Galerius. He was made Caesar (305) by Maximian at the request of his friend and was proclaimed Augustus in 306 by Galerius himself.

When he tried to repress Maxentius rebellion in Rome his troops deserted him and he was forced to flee to Ravenna. There Maximian convinced Severus to abdicate and surrender himself to the rebels. He was brought to Rome and imprisoned at Tres Tabarnae where he was killed when his friend Galerius invaded Italy (307).