Die Fledermaus (The Bat) is an operetta composed by Johann Strauss II to a German libretto by Karl Haffner and Richard Genée.

The original source for Die Fledermaus is Das Gefängnis (The Prison), a farce by German playwright Julius Roderich Benedix (1811–1873). Another source is the French vaudeville play Le réveillon, by Henri Meilhac and Ludovic Halévy, which was first translated by Karl Haffner into a non-musical play to be produced in Vienna. However, the peculiarly French custom of the réveillon (a New Year's Eve supper party) caused problems, which were solved by the decision to adapt the play as a libretto for Johann Strauss, with the réveillon replaced by a Viennese ball. At this point Haffner's translation was handed over for adaptation to Richard Genée, who subsequently claimed not only that he had made a fresh translation from scratch but that he had never even met Haffner.

The operetta premièred on 5 April 1874 at the Theater an der Wien in Vienna and has been part of the regular repertoire ever since:

It was performed in New York under Rudolf Bial at the Stadt Theatre on 21 November 1874. The German première took place at Munich's Gärtnerplatztheater in 1875. Die Fledermaus was sung in English at London's Alhambra Theatre on 18 December 1876, with its score modified by Hamilton Clarke.

The first London performance in German did not take place until 1895. According to the archivist of the Royal Opera House, Covent Garden, "Twenty years after its production as a lyric opera in Vienna, Mahler raised the artistic status of Strauss's work by producing it at the Hamburg Opera House all the leading opera houses in Europe, notably Vienna and Munich, have brightened their regular repertoire by including it for occasional performance."

The role of Eisenstein was originally written for a tenor, but is nowadays frequently sung by a baritone. The role of Orlofsky, a trouser role, is occasionally performed by a tenor or baritone, notably in Vienna.