Gawain (), also known in many other forms and spellings, is a character in Arthurian legend, in which he is King Arthur's nephew and a Knight of the Round Table. The prototype of Gawain is mentioned under the name Gwalchmei in the earliest Welsh sources. He has subsequently appeared in many Arthurian stories in Welsh, Latin, French, English, Scottish, Dutch, German, Spanish, and Italian, notably as the protagonist of the Middle English poem Sir Gawain and the Green Knight. Other tales featuring Gawain as the central character include De Ortu Waluuanii, Diu Crône, Ywain and Gawain, Golagros and Gawane, Sir Gawain and the Carle of Carlisle, L'âtre périlleux, La Mule sans frein, La Vengeance Raguidel, Le Chevalier à l'épée, The Awntyrs off Arthure, The Greene Knight, and The Weddynge of Syr Gawen and Dame Ragnell.

In Arthurian chivalric romance literature, Gawain is usually depicted as King Arthur's closest companion and an integral member of the elite Round Table. In the best-known versions of the legend, he is the son of Arthur's sister Morgause and King Lot of Orkney and Lothian. His younger brothers (or half-brothers) are Agravain, Gaheris, Gareth, and the infamous Mordred. However, his familial relations and upbringing are recorded differently in various accounts. They often involve Gawain unknowingly being raised in foster care in Rome before returning to Britain to reunite happily with his biological relatives. His many children from his numerous wives and lovers include the "Fair Unknown" Gingalain, himself a popular hero of Arthurian romance.

In the early Welsh texts, Gawain is portrayed as a formidable but courteous and compassionate warrior, fiercely loyal to his king and his family. He is known as a friend to young knights, a defender of the poor and the unfortunate, and as the "Maidens' Knight", a rescuer of women as well—in more than one variant of the Castle of Maidens theme. Other notable recurring motifs include his learned healing skills, his special swords that may include the famed Caliburn (Excalibur), and his mighty warhorse named Gringolet. In the later version of his legend, he possesses superhuman strength connected to a day and night cycle, adding to his already outstanding martial prowess and making him an invincible swordsman around noon, when the sun is at its height.

However, Gawain's glowing portrayal diminishes in the Vulgate Cycle, which favours Lancelot and, especially, Galahad. His character turns markedly ignoble in the Post-Vulgate Cycle, and even outright villainous in the Prose Tristan, resulting in his conflicting characterization in Le Morte d'Arthur. While he was originally known for his religious piety, here he becomes a symbol of secular knighthood and thus a major object of criticism by the authors of French prose cycles. Two important plotlines shed light on Gawain's redefined characterization: his being a leader in the family blood feud against the clan of King Pellinore and his initially close friendship with another great knight, Sir Lancelot, which becomes a bitter rivalry when he seeks vengeance for the death of his brothers. In this tradition, featured in Le Morte d'Arthur, Gawain's sinful ways and his unforgiving nature inadvertently lead to the fall of Arthur and the Round Table, and ultimately his own death by Lancelot's hand.