Western Roman emperor who ruled Britain, Gaul, and Spain from AD 383 to 388.

A Spaniard of humble origin, Maximus commanded the Roman troops in Britain against the Picts and Scots. In the spring of 383, Maximus' British troops proclaimed him emperor, and he at once crossed to the European mainland to confront his rival, the Western emperor Gratian. Maximus won over Gratian's advancing troops, Gratian fled but was overtaken and killed in the August of 383AD.

Maximus took up residence at Trier (in present-day Germany) and entered into negotiations with the Eastern emperor, Theodosius I. Since hostile tribes were threatening his eastern frontier, Theodosius decided to recognise Maximus rather than fight a war in the West. Maximus also opened negotiations with Valentinian II, the young ruler who had been co-emperor with Gratian, and made an uneasy peace with him. At this time Maximus elevated his son Flavius Victor to be co-ruler with him, and his elevation was recognised by the other two emperors.

In the summer of 387 Maximus invaded Italy, forcing Valentinian to flee to Thessalonica. War broke out in 388 between Maximus and Theodosius, whose position had been strengthened by a treaty with the Persians. When his troops were defeated near Siscia and at Petovio, in Illyricum (in the Balkans), Maximus was captured and executed.