Geoffrey of Monmouth
 /ˌdʒefri əv ˈmɒnməθ/
  /ˌdʒefri əv ˈmɑːnməθ/
- (c. 1100-54) a Welsh bishop and writer. He wrote Historia Regum Britanniae, a history of the kings of Britain before the arrival of the Anglo-Saxons. It tells the stories of the kings in Shakespeare's plays, such as Lear and Cymbeline, but it is best known as the main source of all the legends about King Arthur.