Ywain /ɪˈweɪn/, also known as Yvain and Owain among many other spellings (Eventus, Ewain[e], Ewein[t], Ivain, Ivan, Iwain[e], Iwein[e], Uwain[e], Yvaine, Yvein, Yvian, Ywan[e], Ywaine, Ywein), is a Knight of the Round Table in the Arthurian legend. Tradition often portrays him as the son of King Urien of Gorre and of either the supernatural figure Modron or the sorceress Morgan. The historical Owain mab Urien, the basis of the literary character, ruled as the king of Rheged in Britain during the late-6th century.

Yvain was one of the earliest characters associated with King Arthur. He was also one of the most popular, starring as the eponymous hero in Chrétien de Troyes' late-12th-century Yvain, the Knight of the Lion and appearing prominently in many later accounts, often accompanied by his fierce pet lion. He remains Urien's son in virtually all literature in which he appears, whereas other Arthurian-legend characters based on historical figures usually lost their original familial connections in romance literature.

Ywain (Yvain) takes his name from Owain mab Urien (Owain son of Urien), a historical figure of the 6th-century Brythonic kingdom of Rheged (in today's northern England and southern Scotland) at the time of the Anglo-Saxon invasion of Britain. His name was recorded in the bardic tradition of Taliesin and became a legendary character in the Welsh Triads, where his father, sister, horse and personal bard are all acclaimed but his wife Penarwan is named one of the "Three Faithless Wives of Britain", along with her sister, Tristan's love Esyllt.

In Geoffrey of Monmouth's Historia Regum Britanniae c. 1136, appearing as Eventus, he is only mentioned in passing, as succeeding his uncle, Auguselus, King of Albany (northern Scotland). According to Heinrich Zimmer, his name could have been derived from the Latin name Eugenius.

In The Dream of Rhonabwy, a Welsh tale associated with the Mabinogion, Owain is one of King Arthur's top warriors who plays a game of chess against him while the Saxons prepare to fight the Battle of Badon. Three times during the game, Owain's men inform him that Arthur's squires have been slaughtering his magical ravens, but when Owain protests, Arthur simply responds, "Your move." Then Owain's ravens retaliate against the squires, and Owain does not stop them until Arthur crushes the chess men. The Saxon leaders arrive and ask for a truce of two weeks, and the armies move on to Cornwall. Rhonabwy, the dreamer of the Dream, awakens, and the reader is left as confused as he is. The Dream of Rhonabwy has never been satisfactorily interpreted.

The Brythonic settlers of Brittany brought much of their insular British culture when they came to the continent, and in the 12th century, updated versions of Breton lais and stories became popular with French audiences. The French poet Chrétien de Troyes wrote the romance Yvain, the Knight of the Lion at the same time he was working on Lancelot, the Knight of the Cart during the 1170s. In it, the eponymous hero Yvain seeks to avenge his cousin Calogrenant who had been defeated by an otherworldly knight beside a magical storm-making fountain in the forest of Brocéliande. Yvain defeats the knight, Esclados, and falls in love with his widow Laudine. With the aid of Laudine's servant Lunete, Yvain wins his lady and marries her, but his cousin Gawain convinces him to embark on chivalric adventure. Yvain's wife assents but demands he return after a set period of time, but he becomes so enthralled in his knightly exploits that he forgets his lady, and she bars him from returning. Yvain goes mad with grief and lives naked in the woods (probably the earliest instance of a hero's mental illness in French literature, which later became a popular motif), but eventually is cured by Morgan and decides to win back his love. A lion he rescues from a dragon proves to be a loyal companion and a symbol of knightly virtue, and helps him complete his quest, which includes defeating the giant Harpins and two demons. In the end, Laudine, rescued from the stake, allows him and his lion to return to her fortress.

Chrétien's Yvain had a large and widespread impact. German poet Hartmann von Aue used it as the basis for his Middle High German court epic Iwein. The author of Owain, or the Lady of the Fountain, one of the Welsh Romances included in the Mabinogion, tells essentially the same story, recasting the work in a Welsh setting (featuring a black lion saved from a serpent). It also exists in several further versions in different languages, including the Middle English Ywain and Gawain.

A mysterious 14th-century so-called Prose Yvain is a text largely unrelated text to Chrétien's poem. It contains only one Yvain-based episode, telling of his rescue of the lion, followed by several more unrelated episodes in which Yvain is no longer main character.