Claim: Guinevere had an affair.

Guinevere -LRB- -LSB- ˈɡwɪnᵻvɪər -RSB- Gwenhwyfar Gwenivar -RRB- , often written as Guenevere or Gwenevere , is , in Arthurian legend , the wife of King Arthur . She first appears as Guanhumara -LRB- with many spelling variants in the manuscript tradition -RRB- in Geoffrey of Monmouth 's pseudo-historical chronicle of British history , the Historia Regum Britanniae , written circa 1136 AD . She is also found in medieval Welsh prose , in the mid-late 12th-century tale Culhwch and Olwen , as Arthur 's wife Gwenhwyfar . In medieval romances , one of the most prominent story arcs is Queen Guinevere 's love affair with her husband 's chief knight , Lancelot . This story first appeared in Chrétien de Troyes 's Lancelot , the Knight of the Cart and became a motif in Arthurian literature , starting with the Lancelot-Grail of the early 13th century and carrying through the Post-Vulgate Cycle and Thomas Malory 's Le Morte d'Arthur . Guinevere and Lancelot 's betrayal of Arthur preceded his eventual defeat at the Battle of Camlann by Mordred .