Geoffrey of Monmouth (c. 1100 – c. 1155) is a cleric and one of the major figures in the development of British historiography and the popularity of tales of King Arthur.

He is best known for his chronicle Historia Regum Britanniae ("History of the Kings of Britain"), which is widely popular in its day and is credited, uncritically, well into the 16th century, being translated into various other languages from its original Latin; but is now considered unreliable as history.