Lucius Tiberius (sometimes Lucius Hiberius, or just simply Lucius) is a Roman Procurator from Arthurian legend appearing first in Geoffrey of Monmouth's Historia Regum Britanniae, though there are passages in Geoffrey's work that give him the title "Emperor".
After Arthur conquers Gaul from the tribune Frollo, word of his great deeds reaches Rome, and Lucius demands that Arthur pay him tribute and recognize him as his sovereign, as had been done by Britain since the time of Julius Caesar. Arthur refuses on the basis that the (mythical) British kings Belinus and Brennius and the British-born Constantine the Great had held Rome in their power before (though this is largely invented by Geoffrey). In retaliation, Lucius gathers armies from Spain and Africa and invades the land of Arthur's allies on the continent. Arthur and the other kings allied with him hurry across the English Channel to do battle with him. Arthur defeats Lucius and, according to Le Morte Darthur, becomes Western Roman Emperor.
The figure of Lucius is clearly mythical, though whether Geoffrey took the character from tradition or completely created him for propagandist purposes is unknown, as is the case with much material in the Historia. Many of the figures associated with him, such as the Eastern and African kings who side with him, appear to be based on figures from Geoffrey's own era.
Geoffrey Ashe theorizes that he was originally Glycerius, whose name was known to have been misspelled as "Lucerius" in texts prior to the writing of the Historia Regum Britanniae, and was further misspelled by Geoffrey of Monmouth as "Lucius Tiberius/Hiberius".
Another theory, proposed by Roger Sherman Loomis, suggests that Lucius is a reflex of the god Lugh, under the name "Llwch Hibernus", which could morph into "Lucius Hiber(i)us".
The legendary Lucius appears in later, particularly English literature such as Thomas Malory's Le Morte Darthur, and the Alliterative Morte Arthure. Roman Emperors defeated by King Arthur appear in French Arthurian literature as well, notably in the Vulgate Cycle.