Magnentius (ruled AD January 18, 350–August 11, 353), was a Roman usurper.

Dissatisfaction amongst the ranks of the Roman army with Constans came to a head with the elevation of Magnentius at Autun on January 18, 350. Constans was abandoned by all except a handful of retainers, and he was slain shortly afterwards by a troop of light cavalry near the Pyrenees.

Magnentius quickly attracted the loyalty of the provinces in Britain, Gaul, and the rest of western Europe, in part because he proved to be far more tolerant towards both Christians and pagans.

The remaining emperor of the family of Constantine the Great, Constantius II broke off his war in the east with Persia, and marched west. Their armies met in the Battle of Mursa in 352; Magnentius led his troops into battle, while Constantius spent the day of battle praying in a nearby church. Despite Magnentius' heroism, his troops were defeated and forced to retreat back to Gaul.

As a result of Magnentius' defeat, Italy ejected his garrisons and rejoined the loyalist cause. Magnentius made a final stand the following year in the Battle of Mons Seleucus , after which he committed suicide.

Following the suppression of Magnentius' rebellion, Constantius commanded a investigation be made to find his followers. The most notorious agent in this search was the primicerius notorarum Paul Catena.