Marcus Aurelius Mausaeus Carausius (d. 293) was a Roman emperor and usurper in Britain and northern Gaul (286 - 293).
Carausius was a Menapian who had proven his ability during Maximian's campaign against the Bagaudae rebels in Gaul in 286. As a result, he was appointed to command a navy based in the English Channel with the responsiblity of eliminating Frankish and Saxon pirates who had been raiding the coast. Apparently, however, Carausius adopted the practice of keeping captured treasure for himself and using it to recruit former pirates into his own fleet. Due to this practice, Maximian came to regard Carausius as a threat and ordered his execution. Carausius learned of this and responded by declaring himself Emperor, and he won the alliegance of the three legions based in Britain, as well as one in northern Gaul.
With the loyalty of these legions and control of his fleet, as well as the support of the Franks and Britons, Carausius was in a strong position. In 289, an invasion of Britain intended to dislodge him failed badly due to storms. Carausius attempted to come to terms with Maximian and his co-emperor Diocletian, but the general Constantius Chlorus was sent to put down the rebellion. It has been speculated (namely, by the historian S.S. Frere) that the rebellion of Carausius endangered Diocletian's vision of a strong, centralized government based on his tetrarchy. In any case, by 293 Constantius had gained control of northern Gaul, including the rebel stronghold and port of Boulogne, on which Carausius was heavily dependent. Constantius also regained the allegiance of the rebellious Gallic legion. Weakened by these setbacks, Carausius was assassinated by his treasurer, Allectus. Monmouth states that Allectus was sent with three legions to kill Carausius and restore Britain to Roman control. Allectus, however, took over as king of the Britons in that same year.
Geoffrey of Monmouth claims that Carausius was given the kingship of the Britons over that of Caracalla, who died many years earlier. At some point in time, Carausius was able to sway the Picts to join him and they turned on the Romans. Caracalla was forced to flee Britain and it remained in Carausius's control for some time. He gave the Picts large tracts of land in Albany where they settled and married Britons.