Nennius — or Nemnius or Nemnivus — was a Welsh monk of the 9th century. He has traditionally been attributed with the authorship of the Historia Brittonum, based on the prologue affixed to that work, This attribution is widely considered a secondary (10th century) tradition.

The Historia Brittonum was highly influential, becoming a major contributor to the Arthurian legend, in particular for its inclusion of events relevant to debate about the historicity of King Arthur. It also includes the legendary origins of the Picts, Scots, St. Germanus and Vortigern, and documents events associated with the Anglo-Saxon invasion of the 7th century as contributed by a Northumbrian document.

Nennius was a student of Elvodugus, commonly identified with the bishop Elfodd of Bangor who convinced British ecclesiastics to accept the Continental dating for Easter, and who died in 809 according to the Annales Cambriae.

Nennius is believed to have lived in the area made up by Brecknockshire and Radnorshire in present-day Powys, Wales.