After the death of Numerian, C. Aurelius Valerius Diocletianus was proclaimed emperor at Nicomedia in AD 284. The following year he defeated Carinus and became sole ruler. Diocletian was hailed as Augustus in AD 286 and he appointed Maximian as his Caesar to handle affairs in Gaul. The next year saw Diocletian's departure for Persia followed by a major diplomatic triumph. While at Sirmium Diocletian began to reorganise the state. He also revived traditional religious cults and assumed the title of Jovius (alluding to Jupiter as his protector). Maximian was to be identified with Hercules, thus establishing the Herculean house of Maximian.

In AD 293 Diocletian established a tetrarchy consisting of two Augusti and two Caesars, each responsible for one part of the empire. Constantius Chlorus and Galerius were appointed Caesars. Diocletian ruled the East and Egypt; Galerius the Balkans; Maximian handled affairs in Italy, Africa and Spain; with Constantius in charge of Gaul and Britain.

Diocletian's reform of provincial, military and financial affairs included a major reform of Roman coinage. In AD 294 a new standard for silver and bronze coinage was implemented and a new reverse type with the Genius of the people (displayed above) distinguished the period of his reign. A new denomination, the follis, was struck in billon or argentiferous bronze and usually displayed on the obverse the emperor laureate. The reverse type most common during the period of the First Tetrarchy, showed the Genius of the people standing, with a modius on his head, a chlamys over his shoulder, holding a patera and cornucopia, with the legend GENIO POPVLI ROMANI. From c. AD 290 a small altar was added in the reverse field. Coinage from this period on heralded the end of regional coinage as it was known - the new standardised imperial coinage struck at decentralised mints conveyed a proud Roman imperial identity and displayed the Genius of the Roman people as a symbol for the entire Roman empire. Other reverse types with Jupiter and Hercules respectively adorned the coins of the Jovian and Herculean houses. Mint signatures became widespread.

In AD 303 Diocletian celebrated his vicennalia at Rome. He returned the next year to Nicomedia, and after suffering a serious illness, decided to abdicate in AD 305. Maximian was persuaded to follow suit and Constantius and Galerius became the new Augusti, with Severus and Maximinus the new Caesars. Following the abdication of Diocletian and Maximian, coinage in their names display the titelature DN (Dominus Noster) in the obverse legend. Diocletian returned to his homeland in Dalmatia where he died in AD 316.

Obv. Diocletian, laureate, facing left, holding sceptre, IMP. C. DIOCLETIANVS AVG.
Rev. Genius standing left, altar to left, GENIO POPVLI ROMANI, PLG in ex.