Marcus (d. 407) was a Roman usurper emperor (406-407) in Roman Britain. He was a soldier in Roman Britain who was proclaimed emperor by the army there some time in 406. He may have risen to power as a reaction to the increasing raids from abroad at a time when the Empire was withdrawing troops from its distant provinces like Britain to protect its heartland. There were too few troops capable of defending Britain at the time, as raiders such as Irish records of raiders like Niall of the Nine Hostages 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 some other, unrecorded crisis. All that is known of his rule is that he did not please the army, and was soon killed by them and replaced with another short-lived usurper, Gratian. In his pseudohistorical Historia Regum Britanniae, Geoffrey of Monmouth tells of a Gracianus Municeps who takes the throne of Britain away from King Dionotus; it is possible he based his characters on the historical Gratian and Marcus.
