Coilus was a legendary king of the Britons during the time of the Roman occupation of Britain as accounted by Geoffrey of Monmouth. He was the son of King Marius and ruled following his father's death.

Of all native kings of Britain, Coilus was the most Romanized. He was brought up in Rome and favored their company in Britain. Throughout his entire reign, he paid Rome its tribute without question. In Britain, he allowed the nobles peace and granted them large gifts. He was succeeded by his only son, St. Lucius.