Marcus Aurelius Antoninus (born Sextus Varius Avitus Bassianus, c. 204 – 11/12 March 222), better known by his nicknames Elagabalus and Heliogabalus, was Roman emperor from 218 to 222, while he was still a teenager. His short reign was notorious for sex scandals and religious controversy. A close relative to the Severan dynasty, he came from a prominent Arab family in Emesa (Homs), Syria, where since his early youth he served as head priest of the sun god Elagabal. After the death of his cousin, the emperor Caracalla, Elagabalus was raised to the principate at 14 years of age in an army revolt instigated by his grandmother Julia Maesa against Caracalla's short-lived successor, Macrinus. He only posthumously became known by the Latinised name of his god.

Later historians suggest Elagabalus showed a disregard for Roman religious traditions and sexual taboos. He replaced the traditional head of the Roman pantheon, Jupiter, with the deity Elagabal, of whom he had been high priest. He forced leading members of Rome's government to participate in religious rites celebrating this deity, presiding over them in person. He married four women, including a Vestal Virgin, in addition to lavishing favors on male courtiers thought to have been his lovers.  He was also reported to have prostituted himself.  His behavior estranged the Praetorian Guard, the Senate and the common people alike. Amidst growing opposition, at just 18 years of age he was assassinated and replaced by his cousin Severus Alexander in March 222. The assassination plot against Elagabalus was devised by Julia Maesa and carried out by disaffected members of the Praetorian Guard.

Elagabalus developed a reputation among his contemporaries for extreme eccentricity, decadence, zealotry and sexual promiscuity. This tradition has persisted; among writers of the early modern age he endured one of the worst reputations among Roman emperors. Edward Gibbon, notably, wrote that Elagabalus "abandoned himself to the grossest pleasures with ungoverned fury".  According to Barthold Georg Niebuhr, "“the name of Elagabalus is branded in history above all others; [...] "Elagabus had nothing at all to make up for his vices, which are of such a kind that it is too disgusting even to allude to them."  An example of a modern historian's assessment is Adrian Goldsworthy's: "Elagabalus was not a tyrant, but he was an incompetent, probably the least able emperor Rome had ever had."