Diadumenian (/daɪˌædʊˈmiːniən/ dy-AD-uu-MEE-nee-ən; Latin: 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.

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.

Diadumenian was born on 14 September 208,[2] named Marcus Opellius Diadumenianus, to Macrinus, the praetorian prefect and future emperor of Berber origin.

[5] Little information survives about Diadumenian, although the details of his physical appearance can be deduced from coinage and a description from the Historia Augusta, which relates that he was "beautiful beyond all others, somewhat tall of stature, with golden hair, black eyes and an aquiline nose; his chin was wholly lovely in its molding, his mouth designed for a kiss, and he was by nature strong and by training graceful".

[7] 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.

This has led to the suggestion, first proposed by the ancient numismatist Curtis Clay, that a large issue of coins was being made for Diadumenian; however, they were quickly melted down when the news of Macrinus' defeat spread.