Marcus Aurelius Antoninus (born Sextus Varius Avitus Bassianus, c. 204 – 13 March 222), better known by his posthumous nicknames Elagabalus (/ˌɛləˈɡæbələs/ EL-ə-GAB-ə-ləs) and Heliogabalus (/ˌhiːliə-, -lioʊ-/ HEE-lee-ə-, -⁠lee-oh-), was Roman emperor from 218 to 222, while he was still a teenager. His short reign was notorious for religious controversy and alleged sexual debauchery. A close relative to the Severan dynasty, he came from a prominent Syrian Arab family in Emesa (Homs), Syria, where he served as the head priest of the sun god Elagabal from a young age. 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.

Elagabalus is largely known from accounts by the contemporary senator Cassius Dio who was strongly hostile to him, Herodian, who likely relied extensively on Dio, and the much later Historia Augusta. The reliability of the accounts of Cassius Dio and the Historia Augusta, particularly their most salacious elements, has been strongly questioned. Elagabalus showed a disregard for Roman religious traditions. He brought the cult of Elagabal (including the large baetyl stone that represented the god) to Rome, making it a prominent part of religious life in the city. He forced leading members of Rome's government to participate in religious rites celebrating this deity, presiding over them in person. According to the accounts of Cassius Dio and the Augusta, he married four women, including a Vestal Virgin, in addition to lavishing favours on male courtiers they suggested to have been his lovers, and prostituted himself. His behaviour 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 posthumous reputation for extreme eccentricity, decadence, zealotry, and sexual promiscuity. 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; [...] "Elagabalus 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". Despite near-universal condemnation of his reign, some scholars write warmly about his religious innovations, including the 6th-century Byzantine chronicler John Malalas, as well as Warwick Ball, a modern historian who described him as "a tragic enigma lost behind centuries of prejudice".

Modern scholars have questioned the accuracy of Roman accounts of his reign, with suggestions that the reports of his salacious behaviour and sexual excess likely reflected a desire to politically discredit him in the immediate aftermath of his death, as well as reflecting Roman stereotypes regarding people from the Orient as effeminate.

Elagabalus was born in 203 or 204, to Sextus Varius Marcellus and Julia Soaemias Bassiana, who had probably married around the year 200 (and no later than 204). Elagabalus's full birth name was probably (Sextus) Varius Avitus Bassianus, the last name being apparently a cognomen of the Emesene dynasty. Marcellus was an equestrian, later elevated to a senatorial position. Julia Soaemias was a cousin of the emperor Caracalla, and there were rumors (which Soaemias later publicly supported) that Elagabalus was Caracalla's child.

Marcellus's tombstone attests that Elagabalus had at least one sibling, possibly a brother, about whom nothing is known. Elagabalus's grandmother, Julia Maesa, was the widow of the consul Julius Avitus Alexianus, the sister of Julia Domna, and the sister-in-law of the emperor Septimius Severus. Other relatives included Elagabalus's aunt Julia Avita Mamaea and uncle Marcus Julius Gessius Marcianus and their son Severus Alexander.

Elagabalus's family held hereditary rights to the priesthood of the sun god Elagabal, of whom Elagabalus was the high priest at Emesa (modern Homs) in Roman Syria as part of the Arab Emesene dynasty. The deity's Latin name, "Elagabalus", is a Latinized version of the Arabic إِلٰهُ الْجَبَلِ Ilāh al-Jabal, from ilāh ("god") and jabal ("mountain"), meaning "God of the Mountain", the Emesene manifestation of Ba'al.

Initially venerated at Emesa, the deity's cult spread to other parts of the Roman Empire in the second century; a dedication has been found as far away as Woerden (in the Netherlands), near the Roman limes. The god was later imported to Rome and assimilated with the sun god known as Sol Indiges in the era of the Roman Republic and as Sol Invictus during the late third century. In Greek, the sun god is Helios, hence Elagabal was later known as "Heliogabalus", a hybrid of "Helios" and "Elagabalus".