The story of the Knight of the Swan, or Swan Knight, is a medieval tale about a mysterious rescuer who comes in a swan-drawn boat to defend a damsel, his only condition being that he must never be asked his name.

The earliest versions, preserved in John of Alta Silva's Dolopathos) do not provide specific identity to this knight, but the Old French Crusade cycle of chansons de geste adapted it to make the Swan Knight (Le Chevalier au Cigne, first version around 1192) the legendary ancestor of Godfrey of Bouillon. The Chevalier au Cigne, also known as Helias, figures as the son of Orient of L'Islefort (or Illefort) and his wife Beatrix in perhaps the most familiar version, which is the one adopted for the late fourteenth century Middle English Cheuelere Assigne. The hero's mother's name may vary from Elioxe (probably a mere echo of Helias) to Beatrix depending on the text, and in a Spanish version, she is called Isomberte.

At a later time, the German poet Wolfram von Eschenbach incorporated the swan knight Loherangrin into his Matter of Britain epic Parzival (first quarter of the 13th century). A German text, written by Konrad von Würzburg in 1257, also featured a Swan Knight without a name. Wolfram's and Konrad's texts were used to construct the libretto for Richard Wagner's opera Lohengrin (Weimar 1850).

Another example of the motif is Brangemuer, the knight who lay dead in a boat tugged by a swan, and whose adventure was taken up by Gawain's brother Guerrehet (Gareth or Gaheris) in the first Continuation to Chrétien de Troyes' Perceval.

The "Swan-Children" appears to have been originally separate from the Godfrey cycle and the Swan Knight story generally. French scholar Gaston Paris identifies four groups of variants, which he classifies usually by the name of the mother of the swan children. The tale in all variants resemble not only such chivalric romances as The Man of Law's Tale and Emaré, but such fairy tales as The Girl Without Hands. It also bears resemblance to the fairy tale The Six Swans, where brothers transformed into birds are rescued by the efforts of their sister.

Scholarship seems to agree with the possibility of a combination of narratives. Geoffrey M. Myers defended that the "swan-children" tale (a narrative of probable Lotharingian origin) is an independent story due to the existence of variants collected from folk tradition, which folklorist Joseph Jacobs considered to be a "well-known Continental folk-tale" (in regards to The Seven Swans (or Ravens)). Thus, according to Sabine Baring Gould, the swan children tale was added to the Knight of the Swan to provide an explanation for the latter.

Similarly, French scholar Gédeon Huet, complementing Gaston Paris's study on the tale, argued that Dolopathos reworked two folktales: "The Brothers Transformed into Birds" (future tale type ATU 451) and "The Sisters Jealous of their Cadette" (future type ATU 707).

German scholar Ernst Tegethoff [de] also claimed that the narrative was a combination of two parts: Genoveva or Calumniated Wife (of possible Germanic origin) and the story of a sister rescuing her brothers from an animal transformation (of possible Celtic origin).