Diadumenian (Marcus Opellius Antoninus Diadumenianus), Roman Caesar (junior emperor), died 218 A.D. was the son of Emperor Macrinus who served his father briefly as Caesar (junior emperor) from 217 to 218 A.D. and as Augustus in 218. Diadumenian was declared to be co-ruler by his father in 218, during the Severan Dynasty, where he is remembered by the name Diadumenianus.
He had little time to enjoy his position or to learn anything from its opportunities because the legions of Syria revolted and declared Elagabalus ruler of the Roman Empire. When Macrinus was defeated on June 8, 218, at Antioch, Diadumenian followed his father's death. According to the Scriptores Historiae Augustae, Diadumenian emulated Macrinus in tyranny. He called upon his father not to spare any who might oppose them or who made plots. His head was cut off and presented to Elagabalus as a trophy.
