Nennius, or Nemnivus, is the name of two shadowy personages traditionally associated with the history of Wales.
The better known of the two is Nennius, the student of Elvodugus. Elvodugus is commonly identified with the bishop Elfoddw of Gwynedd, who convinced the rest of the Welsh portion of Celtic Christianity to celebrate Easter on the same date as the other Catholics in Britain in AD 768, and is later stated by the Annales Cambriae to have died in 809. This Nennius is traditionally stated as having lived in the early 9th century, and is identified in one group of manuscripts of the Historia Brittonum as the author of that work. The careful scholarship of professor David N. Dumville on this text has instead shown that the manuscripts that make this claim come from an exemplar dating to the later eleventh century, far later than the exemplars of other versions of this manuscript – as well as over two hundred years after this Nennius is supposed to have lived.