Diadumenian ( Marcus Opellius Antoninus Diadumenianus; 14 September 208 – June 218) was the son of the Roman emperor Macrinus and served as his co-ruler for a brief time in 218.  Diadumenian became caesar in May 217, shortly after his father's accession to the imperial throne. Elagabalus, a relative of the recently deceased Caracalla, revolted in May of the following year, and Diadumenian was elevated to co-emperor. After Macrinus was defeated in the Battle of Antioch on 8 June 218, Diadumenian was sent to the court of Artabanus IV of Parthia to ensure his safety; however, he was captured and executed along the way. After his death and that of his father, the Senate declared both of them enemies of Rome and had their names struck from records and their images destroyed — a process known in modern scholarship as damnatio memoriae.

 Having served as praetorian prefect under Caracalla, Macrinus participated in a plot to have the Emperor assassinated and exploited the resulting power vacuum to seize the throne for himself on 11 April 217, three days after Caracalla's death. Shortly after, the eight-year-old Diadumenian was elevated to caesar – formalising his position as heir to the throne – at Zeugma, while his guard was escorting him from Antioch to Mesopotamia to join his father. He was also given the name Antoninus, in honour of the Antonine dynasty, at this time. On 16 May 218 a revolt against him and his father was launched in Emesa by Elagabalus, whose mother, Julia Soaemias, was Caracalla's cousin. In order to put down the revolt, Macrinus led his legions to a fort at Apamea. There Macrinus elevated Diadumenian to augustus, making him co-emperor. After Macrinus was defeated by Elagabalus on 8 June 218, at the Battle of Antioch, Macrinus fled north and then to the Bosporus. Before fleeing, he entrusted Diadumenian to loyal servants, instructing them to take him into the Parthian Empire, to the court of Artabanus IV, to ensure his safety. Diadumenian was captured en route in Zeugma and executed in late June   His head was brought to Elagabalus and reportedly kept as a trophy.

Following the demise of both Macrinus and Diadumenian, the Roman Senate quickly proclaimed their support for Elagabalus, declaring the former emperors to be enemies of the state. They were subject to a process known in modern scholarship as damnatio memoriae, with their images and mentions in inscriptions and papyri being destroyed during the reign of Elagabalus.  In an attempt to wipe out all traces of Diadumenian and his father, Elagabalus dated his own reign to the end of that of Caracalla. Surviving busts of Diadumenian are mangled, with the facial features barely being discernible