Elagabalus (Latin: Marcus Aurelius Antoninus Augustus; ca. 203 – 11 March 222), also known as Heliogabalus, was Roman Emperor from 218 to 222. A member of the Severan Dynasty, he was Syrian on his mother's side, the son of Julia Soaemias and Sextus Varius Marcellus. Early in his youth he served as a priest of the god El-Gabal in his hometown, Emesa. Upon becoming emperor he took the name Marcus Aurelius Antoninus Augustus and was called Elagabalus only after his death.

In 217, the emperor Caracalla was assassinated and replaced by his Praetorian prefect, Marcus Opellius Macrinus. Caracalla's maternal aunt, Julia Maesa, successfully instigated a revolt among the Third Legion to have her eldest grandson, Elagabalus, declared emperor in his place. Macrinus was defeated on 8 June 218, at the Battle of Antioch. Elagabalus, barely fourteen years old, became emperor, initiating a reign known mainly for sexual scandal and religious controversy.

During his rule, Elagabalus showed a disregard for Roman religious traditions and sexual taboos. He replaced the traditional head of the Roman pantheon, Jupiter, with a god held in comparable esteem in Syria, Deus Sol Invictus (in Greek: Helios, hence the name Heliogabulus). He forced leading members of Rome's government to participate in religious rites celebrating this deity, rites over which he personally presided. Elagabalus was married as many as five times, lavished favours on courtiers popularly thought to have been his homosexual lovers, employed a prototype of whoopee cushions at dinner parties, and was reported to have prostituted himself in the imperial palace. Reports of his behaviour estranged the Praetorian Guard, the Senate, and the common people alike.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.