The following is a list of usurpers in the Roman Empire —individuals who unsuccessfully claimed and/or attempted to usurp the throne of a ruling emperor (augustus). The ancient term was "tyrant" (Latin: tyrannus), which had negative connotation in and of itself. Usurpation was common during the whole imperial era; virtually all imperial dynasties rose to power through usurpation and conspiracies. The "imperial office" established by Augustus never defined an stable system of succession, and emperors often had to rely solely on military power to survive.
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 Senate or by the legitimate emperor. Emperors marked in bold are those who managed to secure the throne for themselves; all other emperors, unless noted, were murdered in office.
First civil war
Galba (68–69)
Otho (69)
Vitellius (69)
Vespasian (69–79)
Second civil war
Pertinax (193)
Didius Julianus (193)
Septimius Severus (193–211)
Crisis of the Third Century
Macrinus and Diadumenian (217–218)
Elagabalus (218–222), restored the Severan dynasty
Maximinus I (235–238)
Gordian I and Gordian II (238)
Philip (244–249)
Decius (249–251)
Trebonianus Gallus (251–253)
Aemilian (253)
Valerian (253–260) ― captured by the Persians
Claudius Gothicus (268–270) ― died of plague
Quintillus (270)
Aurelian (270–275)
Florian (276)
Probus (276–282)
Carus (282–283) ― died in Persia
Diocletian (284–305)
4th–5th centuries
Constantine I (306–337)
Maxentius (306–312)
Alexander (308–310)
Nepotianus (350)
Magnentius (350–353)
Vetranio (350) ― deposed and spared
Julian II (360–363)
Procopius (365–366)
Magnus Maximus and Victor(383–388)
These last emperors of the West Empire were all accepted by the Senate but never recognized as colleagues by the Emperor of the East.
Constantine III and Constans II (407–411), recognized by Honorius
Priscus Attalus (409–410)
Joannes (423–425)
Petronius Maximus (455)
Avitus (455–456)
Majorian (457-461)
Libius Severus (461–465) ― natural death
Olybrius (472) ― natural death
Glycerius (473–474) ― deposed and spared
Romulus Augustulus (475–476) ― deposed and spared
Unsuccessful usurpers or rebels
1st century rebels
All "usurpers" began as rebels, but not all rebels claimed the imperial titles. These early figures rebelled against the emperor, but most likely did not claim the imperial title.