Diadumenian (Latin: Marcvs Opellivs Antoninvs Diadvmenianvs Avgvstvs) (14/19 September 208 – 218), is the son of the Roman Emperor Macrinus, and serves his father briefly as Caesar (May 217–218) and as Augustus (in 218).

Diadumenian was born on the 14th of September 208 or, according to Historia Augusta, on the 19th of September because he shared the same birthday with the Emperor Antoninus Pius.

His mother was Empress Nonia Celsa, although little is known of her: she is only mentioned by the Historia Augusta.

He was born Marcus Opellius Diadumenianus, but his name was changed and added Antoninus to solidify connection to the family of Marcus Aurelius as done by Caracalla.