The Gypsy Baron (German: Der Zigeunerbaron) is an operetta in three acts by Johann Strauss II which premiered at the Theater an der Wien on 24 October 1885. Its German libretto by Ignaz Schnitzer is based on the unpublished 1883 story Saffi by Mór Jókai. Jokai later published a novel A cigánybáró (English: The Gypsy Baron) in 1885 using an expanded version of this same story.

While an operetta, The Gypsy Baron was a departure from Strauss's earlier more comic and light hearted works; displaying more serious tones both musically and dramatically in what musicologist Andrew Lamb described as "a step in Strauss’s quest for the composition of a genuine opera". During the composer's lifetime, the operetta enjoyed great success internationally. It is considered one of Strauss's three best stage works, along with Die Fledermaus (1874) and Eine Nacht in Venedig (1883).

Strauss was introduced to Jókai's story prior to its publication as a novel in Pest, Hungary in February 1883 by his third wife Adele Deutsch. He dialogued with Jókai about adapting the story into an operetta soon after. Jókai recommended the Hungarian-Austrian writer Ignaz Schnitzer to Strauss as a librettist, and the two men created the work over a two-year period from 1883 through 1885. Its genesis was rather swift and smooth, as Strauss was no stranger to the Hungarian influences apparent in the music score. Many of his earlier works also bore similar themes, such as the polka Éljen a Magyar! ("Long live the Hungarians!"), Op. 332. Originally, Strauss and Schnitzer intended the operetta as an opera but further revisions were made and the idea of a comic opera was conceived.

Strauss' work on the operetta was interrupted in autumn 1883 due to nicotine poisoning and fainting fits and he was to recuperate in Franzensbad. Strauss' work on act 3 of the work was also interrupted when his wife Adele was taken ill and the couple left for Ostend. Not until autumn 1885 was the work finally completed, with Schnitzer making various revisions of the libretto to suit Strauss' style of composing which were not present in the latter's previous stage works.

The Gypsy Baron premiered at the Theater an der Wien on 24 October 1885, and quickly became an international success; enjoying popularity during the remainder of Strauss's life. The composer continued to alter and cut material from the original score of the operetta following its premiere. A 1990 production staged by the Zürich Opera with music director Nikolaus Harnoncourt restored music from the original score under the guidance of Strauss specialist Norbert Linke. This music had not been heard since the original 1885 performances, and Teldec released a recording of this version with Harnoncourt conducting in 1995.

The opera has been regularly performed in the opera houses of Austria and Hungary since its premiere. The Hungarian premiere took place at the People's Theatre (Népszínház) [hu] in Budapest in the spring of 1886 with the composer conducting. In September 1901 the Theater an der Wien mounted a critically lauded revival of the operetta spearheaded by the theater's then director Wilhelm Karczag at a time when Austrian operetta had been largely supplanted in Vienna by operetta works from England. The production cemented The Gypsy Baron's place in the operatic canon and featured the popular actor Alexander Girardi in a reprisal of the role of Kálmán Zsupán from the original production. This staging of the operetta had an impact on Franz Lehár, and heavily influenced the development of his 1905 operetta The Merry Widow.

The operetta was first staged by the Vienna State Opera on December 26, 1910 with conductor Felix Weingartner leading the musical forces. That house staged an elaborate production of the work in 1976. The operetta is part of the regular repertory of the Vienna Volksoper. In 1999 the opera was staged at multiple opera houses in Vienna in celebration of the 100th anniversary of the composer's death; including the Vienna State Opera, the Vienna Volksoper, the Wiener Kammeroper, and the Theater an der Wien.

The US premiere took place on 15 February 1886 at the Casino Theatre on Broadway with tenor William Castle in the title role and soprano Pauline Hall as Saffi. It was first staged by the Metropolitan Opera exactly 20 years later on 15 February 1906 with Andreas Dippel in the title role, Bella Alten as Saffi, Louise Homer as Czipra, Otto Goritz as Kálmán Zsupá, Marie Rappold as Arsena, and Nahan Franko conducting. Both the original Broadway and Met stagings were produced by Heinrich Conried. The Met staged the opera again in 1959 with Nicolai Gedda as Sándor Barinkay, Lisa Della Casa as Saffi, Regina Resnik as Czipra, Walter Slezak as Kálmán Zsupán, Laurel Hurley as Arsena, Paul Franke as Ottokar, Mignon Dunn as Mirabella, and Erich Leinsdorf conducting.