Roman general who, by setting himself up as an independent emperor in Gaul about 258–268 became a rival to the emperor Gallienus.

Postumus and another general, Silvanus, stayed behind in Colonia (Cologne) with Gallienus' son Saloninus after the emperor had left the Rhine River for the Danube about 258. When Silvanus demanded that all booty be handed back to the treasury and its original owners, the reluctant troops proclaimed Postumus emperor, defeating and killing both Silvanus and Saloninus. Postumus successfully defended the Rhine frontier and withstood Gallienus' attempts to recover Gaul (265). At the height of his power, he ruled Gaul, Britain, and Spain. Later he took Victorinus (who succeeded him) as his colleague, perhaps as joint emperor. Postumus was killed in a mutiny of the legion of Mogontiacum (now Mainz, Ger.).