Marcus is the name of a little-recorded pretender to the throne of the Roman Empire during the early 5th century AD.

He was a soldier in Roman Britain who was proclaimed emperor by the the army there some time in AD 406. He may have risen to power as a reaction to the increasing raids from abroad and the imperial need to protect the empire's heartlands rather than distant provinces such as Britain. There were too few troops capable of defending Britain at the time as raiders such as Niall of the Nine Hostages recorded in Irish records show. Local troops, with ties to their home garrisons are likely to have resisted being redeployed to Italy at a time of such instability in Britain. Marcus' elevation may have been a result of this or the result of some other, unrecorded crisis.

All that is known of his rule is that he was soon killed by the army as he did not please them. The army replaced him with another short-lived usurper, named Gratian.