Gracianus Municeps was a legendary king of the Britons after the death of Emperor Magnus Maximus, according to Geoffrey of Monmouth. He is said to have usurped the throne from the rightful king, Dionotus, after word came of Maximus's death.

During the campaigns of Maximus in Rome and Germany, Gracianus was sent to Britain to defeat Wanius and Melga, the kings of the Picts and Huns respectively. Immediately upon arrival, he defeated the armies of both kings and they fled to Ireland. Soon after, word came that Maximus had died by the hands of either a supporter of the late emperor Gratian or a supporter of Gracianus Municeps. Gracianus seized the crown of Britain from king Dionotus and began a reign of terror throughout the island. Not long after he had become king, certain plebs banded together and assassinated Gracianus. He was succeeded by Constantine, the brother of King Aldroenus of Brittany, following a period of caused by continuous invasions.

Historically, the predecessor to Constantine was Gratian on whom Geoffrey's tale was probably based.