The following is a list of usurpers in the Roman Empire. For an overview of the problem and consequences of usurpation, see Roman usurpers. In the Eastern Roman Empire (395–1453), rebellion and usurpation were so notoriously frequent (in the vision of the medieval West, where usurpation was rare) that the modern term "byzantine" became a byword for political intrigue and conspiracy. For usurpation in the Eastern Roman Empire, see List of Byzantine usurpers.

Usurpers who became legitimate emperors

The following individuals began as usurpers, but became the legitimate emperor either by establishing uncontested control of the empire or by confirmation of their position by the Roman Senate or by the legitimate emperor.

First Imperial civil war

Second Imperial civil war

Crisis of the Third Century

Tetrarchy and Later Empire

Western Empire

Most Western emperors were accepted by the Roman Senate (possibly except for Constans II) but almost never recognized as colleagues by the Eastern Emperors. Three of them, (Constantine III, Priscus Attalus, and Constans II), reigned alongside the legitimate Emperor of the West Honorius, who accepted Constantine III as his co-emperor in 409. Upon his recognition, Constantine III appointed his son Constans II co-emperor.

Usurpers not considered legitimate emperors

The following individuals proclaimed themselves emperor (or were proclaimed or appointed as emperor), but are not considered as legitimate emperors because they did not oust the ruling emperor, or did not establish control of the whole empire, or were not accepted by the senate or other imperial colleagues. They are listed here under the emperor whose rule they attempted to usurp. The noted date is the attempted year of usurpation.

Claudius: 41–54

Galba: 68–69

Titus: 79–81

Domitian: 81–96

Marcus Aurelius: 161–180

Septimius Severus: 193–211

Elagabalus: 218–222

Alexander Severus: 222–235

Maximinus Thrax: 235–238

Gordian III: 238–244

Philip the Arab: 244–249

Decius: 249–251

Gallienus: 253–268

Claudius II: 268–270

Aurelian: 270–275

Probus: 276–282

Carus, Carinus, Numerian: 282–284

Diocletian: 284–305

Galerius: 305–311

Constantine I: 309–337

Constantius II: 337–361

Valens: 364–378

Valentinian I: 364–375

Honorius: 395–423

Valentinian III: 423–455

Anthemius: 467–472

Unsuccessful regional usurpers after the fall of Rome (476)