Marcus Opellius Macrinus ( c. 165 – June 218) was Roman emperor from April 217 to June 218, reigning jointly with his young son Diadumenianus. As a member of the equestrian class, he became the first emperor who did not hail from the senatorial class and also the first emperor who never visited Rome during his reign. Before becoming emperor, Macrinus served under Emperor Caracalla as a praetorian prefect and dealt with Rome's civil affairs. He later conspired against Caracalla and had him murdered in a bid to protect his own life, succeeding him as emperor.

Macrinus was proclaimed emperor of Rome by 11 April 217 while in the eastern provinces of the empire and was subsequently confirmed as such by the Senate; however, for the duration of his reign, he never had the opportunity to return to Rome. His predecessor's policies had left Rome's coffers empty and the empire at war with several kingdoms, including Parthia, Armenia and Dacia. As emperor, Macrinus first attempted to enact reform to bring economic and diplomatic stability to Rome. While Macrinus' diplomatic actions brought about peace with each of the individual kingdoms, the additional monetary costs and subsequent fiscal reforms generated unrest in the Roman military.

Caracalla's aunt Julia Maesa took advantage of the unrest and instigated a rebellion to have her fourteen-year-old grandson, Elagabalus, recognized as emperor. Macrinus was overthrown at the Battle of Antioch on 8 June 218 and Elagabalus proclaimed himself emperor with support from the rebelling Roman legions. Macrinus fled the battlefield and tried to reach Rome, but was captured in Chalcedon and later executed in Cappadocia. He sent his son to the care of Artabanus IV of Parthia, but Diadumenian was also captured before he could reach his destination and executed. After Macrinus' death, the Senate declared him and his son enemies of Rome and had their names struck from the records and their images destroyed; the phrase for such a drastic social/historical erasure came to be damnatio memoriae: damnation (of the) memory (of someone).