The Rhinemaidens are the three nixies sisters (Rheintöchter or "Rhine daughters") who appear in Richard Wagner's opera cycle Der Ring des Nibelungen. Their individual names are Woglinde, Wellgunde and Flosshilde, although they are generally treated as a single entity and they act together accordingly. Of the 34 characters in the Ring cycle, they are the only ones who did not originate in the Old Norse Eddas. Wagner created his Rhinemaidens from other legends and myths, most notably the Nibelungenlied which contains stories involving water sprites (nixies) or mermaids of the Danube.

The key concepts associated with the Rhinemaidens in the Ring operas—their flawed guardianship of the Rhine gold, and the condition (the renunciation of love) through which the gold could be stolen from them and then transformed into a means of obtaining world power—are wholly Wagner's own invention, and are the elements that initiate and propel the entire drama.

The Rhinemaidens are the first and the last characters seen in the four-opera cycle, appearing both in the opening scene of Das Rheingold, and in the final climactic spectacle of Götterdämmerung, when they rise from the Rhine waters to reclaim the ring from Brünnhilde's ashes. They have been described as morally innocent, yet they display a range of sophisticated emotions, including some that are far from guileless. Seductive and elusive, they have no relationship to any of the other characters, and no indication is given as to how they came into existence, beyond occasional references to an unspecified "father".

The various musical themes associated with the Rhinemaidens are regarded as among the most lyrical in the entire Ring cycle, bringing to it rare instances of comparative relaxation and charm. The music contains important melodies and phrases which are reprised and developed elsewhere in the operas to characterise other individuals and circumstances, and to relate plot developments to the source of the narrative. It is reported that Wagner played the Rhinemaidens' lament at the piano, on the night before he died in Venice, in 1883.

Alone of the Ring's characters, the Rhinemaidens do not originate from the Poetic Edda or Prose Edda, the Icelandic sources for most of Norse mythology. Water-sprites (German: Nixen) appear in many European myths and legends, often but not invariably in a form of disguised malevolence. Wagner drew widely and loosely from those legends when compiling his Ring narrative, and the probable origin of his Rhinemaidens is in the German Nibelungenlied. In one part of the Nibelungenlied narrative Hagen and Gunther encounter certain mermaids or water sprites (Middle High German: merwîp; mod. Ger.: Meerweib) bathing themselves in the waters of the Danube. Hagen steals their clothes, and seeking their return, the mermaid called Hadeburg gives false prophecy that Hagen and Gunther will find honor and glory when they enter Etzel's kingdom. But afterwards another mermaid, Sigelinde (a name Wagner would adopt again for use elsewhere), tells Hagen her aunt has lied. If they go to Etzel's land, they will die there.

The placement of this scene has several possibilities, but according to Þiðrekssaga, it occurred at the (non-existent) confluence of the Danube and the Rhine. Möringen, where the doomed warriors subsequently ferried across may be Möhringen an der Donau, although Großmehring which is much further east has also been suggested.

This story, itself unrelated to the Ring drama, is echoed by Wagner both in the opening Das Rheingold scene and in the first scene in Act III of Götterdämmerung. Wagner first adapted the story for use in his early libretto of Siegfried's Death (which eventually became Götterdämmerung), introducing three unnamed water-maids (Wasserjungfrauen), and locating them in the Rhine, where they warn Siegfried of his impending death. Later these water-maids became Rhinemaidens (Rheintöchter), and were given individual names: Flosshilde, Wellgunde, and Bronnlinde. As Wagner continued working on his reverse chronology from Siegfried's death, he arrived at what he determined was the initial act of the drama—Alberich's theft of the Rhine gold. Believing that a simple abduction of the unguarded gold would lack dramatic force, Wagner made the Rhinemaidens the guardians of the gold, and he introduced the "renunciation of love" condition. Bronnlinde became Woglinde, probably to avoid confusion with Brünnhilde.

Wagner may also have been influenced by the Rhine River-based German legend of Lorelei, the lovelorn young maiden who drowns herself in the river and becomes a siren, luring fishermen onto the rocks by her singing. Further possible sources lie in Greek mythology and literature. Similarities exist between the maiden guardians in the Hesperides myth and the Rhinemaidens of Das Rheingold; three females guard a highly desired golden treasure that is stolen in the telling of each tale. Wagner was an enthusiastic reader of Aeschylus, including his Prometheus Bound which has a chorus of Oceanids or water nymphs. One author, Rudolph Sabor, sees a link between the Oceanids' treatment of Prometheus and the Rhinemaidens' initial tolerance of Alberich. Just as in Greek myth the Oceanids are the daughters of the titan sea god Oceanus, in Norse mythology—specifically the Poetic Edda—the jötunn (similar to a giant) sea god Ægir has nine daughters. The name of one of these means "wave" (Welle in German) and is a possible source for Wellgunde's name.