Gaheris (/ɡəˈhɛrɪs/ gə-HERR-iss; Old French: Gaheriet[/s/z], Gaheriés, etc.) is a Knight of the Round Table and a relative of King Arthur in the chivalric romance tradition of the Arthurian legend. He is usually described as the third son of one of Arthur's half-sisters and her husband Lot, rulers of either Orkney or Lothian. In the popular version found in Thomas Malory's Le Morte d'Arthur, Gaheris is the son of King Lot of Orkney and Queen Morgause, making him the younger brother of Gawain and Agravain, the elder brother of Gareth, and the half-brother of Arthur's son Mordred.

The character of Gaheris may have originated from the figure of Gawain's sole brother in early Welsh Arthurian tradition, who was later divided into two separate characters, one of whom became known as Malory's Gareth. In German medieval poetry, where no equivalent of Gareth exists, Gaheris appears instead as Gawain's cousin rather than his brother.

In Le Morte d'Arthur, Gaheris is portrayed largely as a supporting character to Arthur's chief nephew, Gawain, with the notable exception of his killing of their mother. His role is more substantial in the French prose cycles that served as Malory's sources, where he becomes the target of murderous sibling rivalry from his elder brother Agravain in the Vulgate Cycle. Ultimately, both in these French texts and in Malory, Gaheris is slain alongside his brother Gareth during Lancelot's rescue of Guinevere, an event that precipitates the downfall of Arthur's realm.

Gaheris and his brother Gareth are thought to have originated from a single figure — the only brother traditionally named for Gwalchmai ap Gwyar, the figure from Welsh mythology generally identified with Gawain. This character, a prince named Gwalchafed (Gwalhafed) or Gwalhauet (Gwalhavet) — Old Welsh for "Hawk of Summer" — ap Gwayr or mab Gwyar, is mentioned in Culhwch and Olwen. He is regarded as the likely common source for both Gaheris and Gareth, assuming that Gawain himself was derived from Gwalchmai. A later French-influenced Welsh romance, Seint Greal, in fact refers to Gwalchmai's brother as Gaharyet.

The names of Gaheris and Gareth, as standardised by Thomas Malory in his compilation Le Morte d'Arthur, are used here for clarity. However, in Malory's sources — the various Old French prose romances — the two are found under a range of variant but similar spellings. Their adventures and character traits are often interchangeable or indistinguishable, and in some manuscripts the two are even conflated within the same text. Due to the numerous confusing French spellings, the International Arthurian Society has described Malory's Gaheris and Gareth as "entirely different characters from Gaheriet and Guerrehes", while also suggesting that Malory may not have intentionally altered them, given the uncertainty surrounding his exact sources.

In continental literature, a Gaheris-like name first appears as Gaheriet (Gaherïet) among King Arthur's knights in the late 12th-century French poem Erec and Enide by Chrétien de Troyes. Gaheriet and Guerehet also appear in Chrétien's later Perceval, the Story of the Grail, described as sons of King Lot and younger brothers of Gawain and Agravain.

In Wolfram von Eschenbach's German poem Parzival, the figure of Gaheriet is represented by Gawain's cousin Gaherjet (Gaherjêt). Der Pleier’s Meleranz mentions Gaharet (also rendered Kaheret in Tandareis and Flordibel), a son of Arthur's sister Anthonje and the unnamed King of Gritenland, presented as one of Gawain's (Gawan) cousins alongside the protagonist Meleranz. As Karjet (Karyet), he also appears in Ulrich von Zatzikhoven's Lanzelet, where he assists Lancelot in rescuing Guinevere from King Valerin's abduction.

The extensive Lancelot-Grail (Vulgate Cycle) prose cycle of the early 13th century is the first known work to feature Gaheris as a major character. In the Prose Lancelot, Gaheris is portrayed as valiant, agile and handsome (with "his right arm longer than the left"), but reticent in speech and prone to excess when angered; he "was the least well-spoken of all his peers." The narrative recounts how the nobles of Orkney (Orcanie), which his father King Lot once ruled, wish to make Gaheris their king, believing him better suited than any of his brothers. Gaheris declines to be crowned until the completion of the Grail Quest. The Prose Merlin similarly depicts him as the finest warrior among Gawain's brothers, at least equal to Gawain himself.