Lancelot du Lac (French for Lancelot of the Lake), alternatively written as Launcelot and other variants, is a popular character in the Arthurian legend's chivalric romance tradition. He is typically depicted as King Arthur's close companion and one of the greatest Knights of the Round Table, as well as a secret lover of Arthur's wife, Guinevere.

In his most prominent and complete depiction, Lancelot is a beautiful orphaned son of King Ban of the lost kingdom of Benoïc. He is raised in a fairy realm by the Lady of the Lake while unaware of his real parentage prior to joining Arthur's court as a young knight and discovering his origins. A hero of many battles, quests and tournaments, and famed as a nearly unrivalled swordsman and jouster, Lancelot soon becomes the lord of the castle Joyous Gard and personal champion of Queen Guinevere, to whom he is devoted absolutely. He also develops a close relationship with Galehaut and suffers from frequent and sometimes prolonged fits of violent rage and other forms of madness. After Lady Elaine seduces him using magic, their son Galahad, devoid of his father's flaws of character, becomes the perfect knight that succeeds in completing the greatest of all quests, achieving the Holy Grail when Lancelot himself fails due to his sins. Eventually, when Lancelot's adulterous affair with Guinevere is publicly discovered, it develops into a bloody civil war that, once exploited by Mordred, brings an end to Arthur's kingdom.

Lancelot's first datable appearance as main character is found in Chrétien de Troyes' 12th-century French poem Lancelot, the Knight of the Cart, which already centered around his courtly love for Guinevere. However, another early Lancelot poem, Lanzelet, a German translation of an unknown French book, did not feature such a motif and the connections between the both texts and their possible common source are uncertain. Later, his character and story was expanded upon Chrétien's tale in the other works of Arthurian romance, especially through the vast Lancelot-Grail prose cycle that presented the now-familiar version of his legend following its abridged retelling in Le Morte d'Arthur. Both loyal and treasonous, Lancelot has remained a popular character for centuries and is often reimagined by modern authors.

There have been many theories regarding the origins of Lancelot as an Arthurian romance character. In those postulated by Ferdinand Lot and Roger Sherman Loomis, Lancelot's figure is related to Llenlleog (Llenlleawc), an Irishman in the early Arthurian Welsh tale Culhwch and Olwen (which associates him with the "headland of Gan[i]on"), and the Welsh hero Llwch Llawwynnauc (most likely a version of the euhemerised Irish deity Lug[h] Lonbemnech, with "Llwch" meaning "Lake" in Welsh), possibly via a now-forgotten epithet such as Lamhcalad, suggesting that they are the same figure; their similarities beyond the name include wielding a sword and fighting for a cauldron in Culhwch and Preiddeu Annwn. Loomis also linked Lancelot to the Welsh mythological hero Lleu Llaw Gyffes, while T. Gwynn Jones claimed links between Lancelot and Eliwlod (Eliwlad), a nephew of Arthur in the Welsh legend. Proponents of the Scythian origins of Arthurian legend have speculated that an early form might have been Alanus-à-Lot, that is "Alan of the river Lot", and those looking for clues in classical antiquity see elements of Lancelot in the Ancient Greek mythical figures of Askalos and Mopsus (Moxus).

Alfred Anscombe proposed in 1913 that the name "Lancelot" came from Germanic *Wlancloth, with roots in the Old English wlenceo (pride) and loða (cloak), in connection with Vinoviloth, the name of a Gothic chief or tribe mentioned in the 6th-century Getica. According to more recent authors, such as Norma Lorre Goodrich, the name, if not just an invention of the 12th-century French poet Chrétien de Troyes, may have been derived from Geoffrey of Monmouth's character Anguselaus, probably a Latinised name of Unguist, the name of a son of the 6th-century Pictish king Forgus; when translated from Geoffrey's Latin into Old French, it would become Anselaus. Other 6th-century figures proposed in modern times as candidates for the prototype of Lancelot include the early French saint Fraimbault de Lassay; Maelgwn, king of Gwynedd; and Llaennog (Llaenauc), father of Gwallog, king of Elmet.

Lancelot may have been the hero of a popular folk tale that was originally independent but was ultimately absorbed into the Arthurian tradition. The theft of an infant by a water fairy, the appearance of the hero at a tournament on three consecutive days in three different disguises, and the rescue of a queen or princess from an Otherworld prison are all features of a well-known and widespread tale, variants of which are found in numerous examples collected by Theodore Hersart de la Villemarqué in his Barzaz Breiz, by Emmanuel Cosquin in his Contes Lorrains, and by John Francis Campbell in his Tales of the West Highlands. As for his name, Lancelot may be a variant of the French name Lancelin (the word likely meaning javelin in Old French) as proposed by Gaston Paris in 1881, later supported by Rachel Bromwich. It is also possibly derived from the Old French word L'Ancelot, meaning "Servant" (the hypothesis first put forward by de la Villemarqué in 1842); Lancelot's name is actually written this way in several manuscripts. It is furthermore reminiscent of an uncommon Saxon name Wlanc, meaning "The Proud One".

Stephen Pow has recently argued that the name "Lancelot" represents an Old French pronunciation of Hungarian "László" (Ladislaus) as inspired by the historical King Ladislaus I of Hungary. In the early 1180s, King Béla III of Hungary was pursuing Ladislaus' canonization as a saint (approved 1192) and a marriage alliance with France through Margaret of France (whom he married 1186). Margaret was the half-sister of Chrétien's patroness, Marie de Champagne, and the creation of Lancelot would thus meant to honor the Hungarian king around the time of his marriage to a member of the French royal house.

Lancelot's name appears third on a list of knights at King Arthur's court in the earliest known work featuring him as a character: Chrétien de Troyes' Old French poem Erec and Enide (1170). The fact that his name follows Gawain and Erec indicates the presumed importance of the knight at court, even though he did not figure prominently in Chrétien's tale. Lancelot reappears in Chrétien's Cligès, in which he takes a more important role as one of the knights that Cligès must overcome in his quest.