Rusalka, Op. 114, is an opera ('lyric fairy tale') by Antonín Dvořák (1841-1904). The Czech libretto was written by the poet Jaroslav Kvapil (1868–1950) based on the fairy tales of Karel Jaromír Erben and Božena Němcová. A rusalka is a water sprite from Slavic mythology, usually inhabiting a lake or river. Rusalka is one of the most successful Czech operas, and represents a cornerstone of the repertoire of Czech opera houses.
 

Dvořák had played viola for many years in pit orchestras in Prague (Estates Theatre from 1857 until 1859 while a student, then from 1862 until 1871 at the Provisional Theatre). He thus had direct experience of a wide range of operas by Mozart, Weber, Rossini, Lortzing, Verdi, Wagner and Smetana. Rusalka was the ninth opera Dvořák composed.
 

For many years unfamiliarity with Dvořák's operas outside the Czech lands helped reinforce a perception that composition of operas was a marginal activity, and that despite the beauty of its melodies and orchestral timbres Rusalka was not a central part of his output or of international lyric theatre. In recent years it has been performed more regularly by major opera companies. In the five seasons from 2008 to 2013 it was performed by opera companies worldwide far more than all of Dvořák's other operas combined.
 

The most popular excerpt from Rusalka is the "Song to the Moon" ("Měsíčku na nebi hlubokém") from act 1, which is often performed in concert and recorded separately. It has also been arranged for violin and used on film sound tracks.

Kvapil's libretto, based on Erben's and Božena Němcová's work, was written before he had any contact with the composer. The plot contains elements which also appear in The Little Mermaid by Hans Christian Andersen and in Undine by Friedrich de la Motte Fouqué, and has been described as a "sad, modern fairy tale", in a similar vein to his previous play, Princessa Pampeliška. The libretto was completed by 1899, when Kvapil began looking for composers interested in setting his text. His composer friends were engaged with other works, but mentioned that Dvořák was looking for a project. The composer, always interested in Erben's stories, read the libretto and composed his opera quite rapidly, with the first draft begun on 22 April 1900 and completed by the end of November. Coming after his four symphonic poems inspired by the folk-ballads of Erben of 1896–97, Rusalka may be viewed as the culmination of Dvořák's exploration of a "wide variety of drama-creating musical techniques".