Gaheris /ɡəˈhɛrɨs/ is a character in the Arthurian legend, a nephew of King Arthur and a Knight of the Round Table. As son of Arthur's sister or half-sister Morgause and her husband Lot, King of Orkney and Lothian, he is a brother of Gawain, Agravaine and Gareth and half-brother of Mordred.

In Thomas Malory's Le Morte d'Arthur, Gaheris is squire to his elder brother Gawain, whose fiery temper he helps moderate, before being knighted himself. He participates in the revenge killing of King Pellinore, his father's slayer, and Sir Lamorak, Pellinore's son and his mother's lover. More notorious is his beheading of his own mother, Morgause, after catching her in flagrante delicto with the youthful and handsome Lamorak, who meanwhile escapes. Believing him to be their mother's murderer, Lamorak is hunted down by all of the brothers except Gareth, and Mordred stabs him in the back. Since, in Malory, Lamorak is greater than any knight except Lancelot and Tristram, the Orkney brothers' act of revenge is deemed cowardly and a blot on their honour. When Arthur and the brothers discover Gaheris is Morgause's slayer, he is banished from court.