Ruddigore; or, The Witch's Curse, originally called Ruddygore, is a comic opera in two acts, with music by Arthur Sullivan and libretto by W. S. Gilbert. It is one of the Savoy Operas and the tenth of fourteen comic operas written together by Gilbert and Sullivan. It was first performed by the D'Oyly Carte Opera Company at the Savoy Theatre in London on 22 January 1887.

The first night was not altogether a success, as critics and the audience felt that Ruddygore (as it was then called) did not measure up to its predecessor, The Mikado. After some changes, including respelling the title, it achieved a run of 288 performances. The piece was profitable, and the reviews were not all bad. For instance, The Illustrated London News praised the work of both Gilbert and, especially, Sullivan: "Sir Arthur Sullivan has eminently succeeded alike in the expression of refined sentiment and comic humour. In the former respect, the charm of graceful melody prevails; while, in the latter, the music of the most grotesque situations is redolent of fun."

There were further changes and cuts, including a new overture, when Rupert D'Oyly Carte revived Ruddigore after the First World War. Although never a big money-spinner, it remained in the repertoire until the company closed in 1982. A centenary revival at Sadler's Wells in London restored the opera to almost its original first-night state. In 2000, Oxford University Press published a scholarly edition of the score and libretto, edited by Sullivan scholar David Russell Hulme. This restores the work as far as possible to the state in which its authors left it and includes a substantial introduction that explains many of the changes, with appendices containing some music deleted early in the run. After the expiration of the British copyright on Gilbert and Sullivan works in 1961, and especially since the Sadler's Wells production and recording, various directors have experimented with restoring some or all of the cut material in place of the 1920s D'Oyly Carte version.

After The Mikado opened in 1885, Gilbert, as usual, promptly turned his thoughts to finding a subject for a next opera. Some of the plot elements of Ruddigore had been introduced by Gilbert in his earlier one-act opera, Ages Ago (1869), including the tale of the wicked ancestor and the device of the ancestors stepping out of their portraits. Heinrich Marschner's 1828 opera, Der Vampyr, involves a Lord Ruthven who must abduct and sacrifice three maidens or die. Locals claim that the Murgatroyd ancestors in Ruddigore are based on the Murgatroyd family of East Riddlesden Hall, West Yorkshire. According to his biographers, Sidney Dark and Rowland Grey, Gilbert also drew on some of his earlier verse, the Bab Ballads, for some plot elements. The song "I know a youth who loves a little maid" can be traced back to the Bab Ballad "The Modest Couple", in which the very shy and proper Peter and Sarah are betrothed but are reluctant to shake hands or sit side by side. Sir Roderic's Act II song "When the night wind howls" had its forerunner in one of Gilbert's verses published in Fun magazine in 1869:

Fair phantom, come! The moon's awake,
The owl hoots gaily from its brake,
The blithesome bat's a-wing.
Come, soar to yonder silent clouds;
The ether teems with peopled shrouds:
We'll fly the lightsome spectre crowds,
Thou cloudy, clammy thing!

The opera also includes and parodies elements of melodrama, popular at the Adelphi Theatre. There is a priggishly good-mannered poor-but-virtuous heroine, a villain who carries off the maiden, a hero in disguise and his faithful old retainer who dreams of their former glory days, the snake-in-the-grass sailor who claims to be following his heart, the wild, mad girl, the swagger of fire-eating patriotism, ghosts coming to life to enforce a family curse, and so forth. But Gilbert, in his customary topsy-turvy fashion, turns the moral absolutes of melodrama upside down: The hero becomes evil, the villain becomes good, and the virtuous maiden changes fiancés at the drop of a hat. The ghosts come back to life, foiling the curse, and all ends happily.

Sullivan delayed in setting Ruddigore to music through most of 1886. He had committed to a heavy conducting schedule and to compose a cantata, The Golden Legend, for the triennial Leeds Music Festival in October 1886. He also was squiring Fanny Ronalds to numerous social functions. Fortunately, The Mikado was still playing strongly, and Sullivan prevailed on Gilbert to delay production of Ruddigore. He got down to business in early November, however, and rehearsals began in December. During the Act II ghost scene, it would be impossible for the cast to see Sullivan's baton when the stage was darkened for the Ancestors' reincarnation. A technological solution was found: Sullivan used a glass tube baton containing a platinum wire that glowed a dull red.

The opera encountered some criticism from audiences at its opening on 22 January 1887, and one critic wondered if the libretto showed "signs of the failing powers of the author". After a run shorter than any of the earlier Gilbert and Sullivan operas premiered at the Savoy except Princess Ida, Ruddigore closed in November 1887 to make way for a revival of H.M.S. Pinafore. To allow the revival of the earlier work to be prepared at the Savoy, the last two performances of Ruddigore were given at the Crystal Palace, on 8 and 9 November. It was not revived in the lifetimes of the composer or author.