The earliest attestation of the name Arthur is in the early 9th century Welsh-Latin text Historia Brittonum, where it refers to a circa 5th century Romano-British general who fought against the invading Saxons, and who later gave rise to the famous King Arthur of medieval legend and literature.

[4] The Irish borrowed the name by the late 6th century (either from an early Archaic Welsh or Cumbric form Artur), producing Old Irish Artúr (Latinized as Arturius by Adomnán in his Life of St. Columba, written circa 697–700),[3][5] The earliest historically attested bearer of the name is a son or grandson of Áedán mac Gabráin (died 609).

[11][12][13] Nevertheless, according to etymologist Kemp Malone "the theory that derives Arthur from Artorius is left unsupported by any evidence worthy of the name".

[7] According to the linguist and Celticist Stefan Zimmer, it is possible that Artorius has a Celtic origin, being a Latinization of the hypothetical name *Artorījos, derived from the patronym *Arto-rīg-ios, meaning "Son of the Bear/Warrior-King".

[27] This form, Arcturus would have become Art(h)ur when borrowed into Welsh, and its brightness and position in the sky led people to regard it as the "guardian of the bear" and the "leader" of the other stars in Boötes.