The coin bearing the name of Sponsian and his portrait was found more than 300 years ago in Transylvania, once a far-flung outpost of the Roman empire.
Believed to be a fake, it had been locked away in a museum cupboard.
Now scientists say scratch marks visible under a microscope prove that it was in circulation 2,000 years ago.
Prof Paul Pearson University College London, who led the research, told BBC News that he was astonished by the discovery.
“What we have found is an emperor. He was a figure thought to have been a fake and written off by the experts.
“But we think he was real and that he had a role in history.”