Diocletian (245–313 A.D.), a Roman emperor. His full name was Gaius Aurelius Valerius Diocletianus. He was one of the great Roman emperors. Coming to power when the empire was experiencing civil war and in a state of financial collapse, Diocletian restored peace and introduced financial reforms. His division of the empire for administrative purposes was the first step toward the separate Eastern (Byzantine) and Western (Roman) empires. It also marked the end of the Roman Senate as an imperial governing body.

Diocletian is also remembered as the emperor who inaugurated the last great persecution of Christians.

Diocletian was an Illyrian of humble birth. He became a soldier and rose to become commander of the imperial bodyguard. In 284, after the murder of three emperors in three years, he was chosen emperor by his fellow soldiers. A brief civil war secured him the throne.

To help bring the empire under control, Diocletian made Maximian, a fellow Illyrian, co-emperor in 286. Problems were so widespread, however, that in 293 Diocletian created a tetrarchy(rule by four). He and Maximian, holding the title of Augustus, each took a subordinate emperor with title of Caesar, whom they adopted as successors. The Caesar of Diocletian was Galerius; of Maximian, Constantius (father of Constantine the Great). All four rulers were declared by Diocletian to be divine. The four seats of government were in Asia Minor, the Baltkans, Gaul (western Europe), and Milan, Italy. Rome, the traditional capital, ruled only its own countryside.

With the four emperors as generals, Roman legions won victories on all fronts. Diocletian took provincial government out of the hands of the army, reformed the currency, and established economic controls. Finally, he moved against the Christians, who refused to worship an emperor or the empire's official gods. Severe penalties were imposed by a series of edicts issued during 303–04.

In 305 Diocletian and Maximian abdicated. Government fell into immediate confusion, and Maximian returned to power, but Diocletian refused to do so.