Guinevere ( (listen) GWIN-iv-eer; Welsh: Gwenhwyfar pronunciation ; Breton: Gwenivar, Cornish: Gwynnever), also often written in Modern English as Guenevere or Guenever, was, according to Arthurian legend, an early-medieval queen of Great Britain and the wife of King Arthur. First mentioned in popular literature in the early 12th century, nearly 700 years after the purported times of Arthur, Guinevere has since been portrayed as everything from a fatally flawed, villainous and opportunistic traitor to a noble and virtuous lady. Many records of the legend also feature the variably recounted story of her abduction and rescue as a major part of the tale.

The earliest datable appearance of Guinevere is in Geoffrey of Monmouth's pseudo-historical British chronicle Historia Regum Britanniae, in which she is seduced by Mordred during his ill-fated rebellion against Arthur. In a later medieval Arthurian romance tradition from France, a prominent story arc is the queen's tragic love affair with her husband's chief knight and trusted friend, Lancelot, indirectly causing the death of Arthur and the downfall of the kingdom. This motif had originally appeared in nascent form in the poem Lancelot prior to its vast expansion in the prose cycle Lancelot-Grail, consequently forming much of the narrative core of Thomas Malory's seminal English compilation Le Morte d'Arthur. Other themes found in Malory and other texts include Guinevere's usual barrenness, the scheme of Guinevere's evil twin to replace her, and the particular hostility displayed towards Guinevere by her sister in law Morgan.

Guinevere has continued to be a popular character featured in numerous adaptations of the legend since the 19th-century Arthurian revival. Many modern authors, usually following or inspired by Malory's telling, typically still show Guinevere in her illicit relationship with Lancelot as defining her character. In much of more recent Arthuriana, however, she assumes more active roles than in her medieval depictions, increasingly even being cast as protagonist.