The Crisis of the Third Century is a commonly applied name for the crumbling and near collapse of the Roman Empire between 235 and 284. It is also called the period of the "military anarchy".

After Augustus declared an end to the Civil Wars of the 1st century BC, the Empire had enjoyed a period of limited external invasion, internal peace and economic prosperity (the Pax Romana). In the 3rd century, however, the Empire underwent military, political and economic crises and began to collapse. There was constant barbarian invasion, civil war, and hyperinflation. Part of the problem had its origins in the nature of the Augustan settlement. Augustus, intending to downplay his position, had not established rules for the succession of emperors.

Already in the 1st and 2nd century, disputes about the succession had led to short civil wars, but in the 3rd century these civil wars became a constant factor, as no single candidate succeeded in quickly overcoming his opponents or holding on to the Imperial position for very long. Between 235 and 284 no fewer than 25 different emperors ruled Rome (the Soldier-Emperors). All but two of these emperors were either murdered or killed in battle. The organisation of the Roman military, concentrated on the borders, could provide no remedy against foreign invasions once the invaders had broken through. A decline in citizens' participation in local administration forced the Emperors to step in, gradually increasing the central government's responsibility.

This period ended with the accession of Diocletian. Diocletian, either by skill or sheer luck, solved many of the acute problems experienced during this crisis. However, the core problems would remain and cause the eventual destruction of the western empire. The transitions of this period mark the beginnings of Late Antiquity and the end of Classical Antiquity.