The Fisher King (French: Roi Pêcheur; Welsh: Brenin Pysgotwir; Cornish: Pyscador Myghtern; Breton: Roue ar Peskataer) is a figure in Arthurian legend, the last in a long line of British kings tasked with guarding the Holy Grail. The Fisher King is both the protector and physical embodiment of his lands, but a wound renders him impotent and his kingdom barren. Unable to walk or ride a horse, he is sometimes depicted as spending his time fishing while he awaits a "chosen one" who can heal him. Versions of the story vary widely, but the Fisher King is typically depicted as being wounded in the groin, legs, or thigh. The healing of these wounds always depends upon the completion of a hero-knight's task.

Most versions of the story contain the Holy Grail and the Lance of Longinus as plot elements. In some versions, a third character is introduced; this individual, unlike the hero-knight archetype, is ignorant of the King's power, but has the ability to save the king and land, or to doom it. Variations of this third party produce divergent legends.

As a literary character, the Fisher King originates in Chrétien de Troyes' unfinished writings of the adventures of Perceval. Many authors have endeavoured to complete and extend the work, resulting in various continuations. Major sources of the legend include Chrétien's Li Contes del Graal; Perceval, ou Le Conte du Graal (c. 1160–1180), Wauchier de Denain's First Continuation (c. 1180–1200), Robert de Boron's Didot-Perceval (c. 1191–1202), Peredur son of Efrawg (c. 1200), Perlesvaus (c. 1200), Wolfram von Eschenbach's Parzival (c. 1217), and Thomas Malory's Morte D'Arthur (c. 1400). The Fisher King can be the Grail Knight's father (as in Chrétien's original) or his other relative (uncle, cousin, grandfather).

The Fisher King first appears in Chrétien de Troyes's Perceval, the Story of the Grail in the late 12th century, but the character's roots may lie in Welsh mythology. He may be derived from the figure of Brân the Blessed in the Mabinogion. In the Second Branch, Bran has a cauldron that can resurrect the dead, albeit imperfectly; thus those revived cannot speak. He gives this cauldron to the king of Ireland as a wedding gift for him and Bran's sister Branwen. Later, Bran wages war on the Irish and is wounded in the foot or leg, and the cauldron is destroyed. He asks his followers to sever his head and take it back to Britain, and his head continues talking and keeps them company on their trip. The group lands on the island of Gwales, where they spend 80 years in a castle of joy and abundance, but eventually they leave and bury Bran's head in London. This story has analogues in two other important Welsh texts: the Mabinogion tale "Culhwch and Olwen", in which King Arthur's men must travel to Ireland to retrieve a magical cauldron, and the poem The Spoils of Annwn, which speaks of a similar mystical cauldron sought by Arthur in the otherworldly land of Annwn.

The Welsh Romance Peredur son of Efrawg is based on Chrétien or derived from a common original, but it contains several prominent deviations and lacks a Grail. The character of the Fisher King appears (though he is not called such) and presents Peredur with a severed head on a platter. Peredur later learns that he was related to that king, and that the severed head was that of his cousin, whose death he must avenge by defeating the Nine Witches.

The Fisher King is a character in Chrétien's Perceval (1180) which is the first of a series of stories and texts on the subject of Perceval and the Grail.

Parzival was written in 1210 by Wolfram von Eschenbach, thirty years after Perceval. Although a different work, it is strikingly similar to Perceval. The story revolves around the Grail Quest and once again the main character is Percival or Parzival. As in Perceval, Eschenbach's story does not have Parzival ask the healing question initially, which results in him Questing for years. However, Eschenbach's Parzival differs from Chrétien's Perceval in three major ways. Firstly, the Fisher King is no longer nameless and is called Anfortas. Secondly, Eschenbach thoroughly describes the nature of the wound; it is a punishment for wooing a woman who is not meant for him (every Grail keeper is to marry the woman the Grail determines for him), and it causes him immense pain. Lastly, Parzival comes back to cure the Fisher King. Parzival, unlike its predecessor Perceval, has a definitive ending.

The Fisher King's next development occurred around the end of the 12th century in Robert de Boron's Joseph d'Arimathie [fr], the first work to connect the Grail with Jesus. Here, the "Rich Fisher" is called Bron, a name similar enough to Bran to suggest a relationship, and said to be the brother-in-law of Joseph of Arimathea, who had used the Grail to catch Christ's blood before laying him in the sepulchre. Joseph founds a religious community that travels eventually to Britain and entrusts the Grail to Bron (who is called the "Rich Fisher" because he catches a fish eaten at the Grail table). Bron founds the line of Grail keepers that eventually includes Perceval.