Nennius is a mythical prince of Britain at the time of Julius Caesar's invasions of Britain (55–54 BC). His story appears in Geoffrey of Monmouth's History of the Kings of Britain (1136), a work whose contents are now considered largely fictional. In Middle Welsh versions of Geoffrey's Historia he was called Nynniaw.
In Geoffrey's story, Nennius is said to have fought Caesar in personal combat and taken his sword, which he used to kill many Romans. In the Tudor and Jacobean eras he became an emblem of British patriotism.