Geoffrey of Monmouth (Latin: Galfridus Monemutensis, Galfridus Arturus, Galfridus Artur, Welsh: Gruffudd ap Arthur, Sieffre o Fynwy) (c. 1100 – c. 1155) was 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 was widely popular in its day and was credited, uncritically, well into the 16th century, being translated into various other languages from its original Latin.

Geoffrey was probably born some time between 1100 and 1110 in Wales or the Welsh Marches. He must have reached the age of majority by 1129, when he is recorded as witnessing a charter.

In his Historia, Geoffrey refers to himself as Galfridus Monumetensis, "Geoffrey of Monmouth", which indicates a significant connection to Monmouth, Wales, and which may refer to his birthplace. Geoffrey's works attest to some acquaintance with the place-names of the region. To contemporaries, Geoffrey was known as Galfridus Artur(us) or variants thereof. The "Arthur" in these versions of his name may indicate the name of his father, or a nickname based on Geoffrey's scholarly interests.

Coordinates: 51°49′N 2°43′W / 51.81°N 2.72°W / 51.81; -2.72

Monmouth ( /ˈmɒnməθ/ MON-məth; Welsh: Trefynwy meaning "town on the Monnow") is a traditional county town in southeast Wales. It is situated where the River Monnow meets the River Wye, within 2 miles (3.2 km) of the border with England. The town is 36 miles (58 km) north-east of Cardiff, and 127 miles (204 km) west of London. It is within the Monmouthshire local authority, and the parliamentary constituency of Monmouth. According to the 2001 census, its population was 8,877.

The town was the site of a small Roman fort, Blestium, and became established after the Normans built a castle here after 1067. Its mediaeval stone gated bridge is the only one of its type remaining in Britain. The castle later came into the possession of the House of Lancaster, and was the birthplace of King Henry V in 1387. In 1536, it became the county town of Monmouthshire.

Monmouth later became a tourist centre at the heart of the Wye Valley, as well as a market town. It now acts as a shopping and service centre, and as a focus of educational and cultural activities, for its surrounding rural area, and is linked by the A40 road to the M4 motorway at Newport and the M50 at Ross-on-Wye.

King Arthur is a legendary British leader of the late 5th and early 6th centuries, who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the early 6th century. The details of Arthur's story are mainly composed of folklore and literary invention, and his historical existence is debated and disputed by modern historians. The sparse historical background of Arthur is gleaned from various sources, including the Annales Cambriae, the Historia Brittonum, and the writings of Gildas. Arthur's name also occurs in early poetic sources such as Y Gododdin.

The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). Some Welsh and Breton tales and poems relating the story of Arthur date from earlier than this work; in these works, Arthur appears either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. How much of Geoffrey's Historia (completed in 1138) was adapted from such earlier sources, rather than invented by Geoffrey himself, is unknown.