King Arthur, or The British Worthy (Z. 628), is a semi-opera in five acts with music by Henry Purcell and a libretto by John Dryden. It was first performed at the Queen's Theatre, Dorset Garden, London, in late May or early June 1691.

The plot is based on the battles between King Arthur's Britons and the Saxons, rather than the legends of Camelot (although Merlin does make an appearance). It is a Restoration spectacular, including such supernatural characters as Cupid and Venus plus references to the Germanic gods of the Saxons, Woden, Thor, and Freya. The tale centres on Arthur's endeavours to recover his fiancée, the blind Cornish Princess Emmeline, who has been abducted by his arch-enemy, the Saxon King Oswald of Kent.

King Arthur is a "dramatick opera" or semi-opera: the principal characters do not sing, except if they are supernatural, pastoral or, in the case of Comus and the popular Your hay it is mow'd, drunk. Secondary characters sing to them, usually as diegetic entertainment, but in Act 4 and parts of Act 2, as supernatural beckonings. The singing in Act 1 is religious observance by the Saxons, ending with their heroic afterlife in Valhalla. The protagonists' parts are taken by actors, as a great deal of King Arthur consists of spoken text. This was normal practice in 17th century English opera. King Arthur contains some of Purcell's most lyrical music, using adventurous harmonies for the day.

Dryden probably wrote the original libretto for King Arthur in 1684 to mark the 25th anniversary of King Charles II's Restoration the following year. The original text of King Arthur no longer exists but it was to be in three acts with an allegorical prologue. For unknown reasons Dryden abandoned his intention to have the whole work set to music and developed the prologue into another opera, Albion and Albanius, a collaboration with the Catalan composer Louis Grabu. However, Charles II died in February 1685 and Albion and Albanius was first inauspiciously performed in June 1685 during the Monmouth Rebellion. It was a failure and Dryden shelved any plans he had for the rest of the King Arthur libretto.

In the meantime, England entered a turbulent period in its history. After the Catholic James II took the throne, Dryden too converted to Catholicism. When the Protestant William III overthrew James in the Glorious Revolution in 1688, Dryden refused to renounce his faith and so lost his job as poet laureate to his rival Thomas Shadwell. Purcell's career had also suffered after the death of the music-loving Charles II. With their sources of royal patronage gone, both playwright and composer were looking to make money as freelance professionals and the London stage offered attractive opportunities.

In 1690, the theatre manager Thomas Betterton decided to risk putting on another operatic work, the first since the ill-fated Albion and Albanius. This was the semi-opera Dioclesian (1690), an adaptation of a play by Beaumont and Fletcher. Purcell's music for the production and the lavish staging made it a triumph and Betterton was eager for another such success. He persuaded Dryden to dust off and revise the libretto for King Arthur so Purcell could set it. The two had already collaborated on stage works (Dryden had written the prologue for Dioclesian and Purcell the incidental music for Dryden's comedy Amphitryon) and Dryden was effusive in his praise of Purcell's musical abilities.

In his preface to the printed edition, Dryden explained he had had to adapt the libretto to the changed political circumstances of 1691: "But not to offend the present Times, nor a Government which has hitherto protected me, I have been oblig'd so much to alter the first Design, and take away so many Beauties from the Writing, that it is now no more what it was formerly..." He also made alterations to suit Purcell's musical needs: "the Numbers of Poetry and Vocal Musick, are sometimes so contrary, that in many places I have been oblig'd to cramp my Verses, and make them rugged to the Reader, that they may be harmonious to the Hearer: Of which I have no Reason to repent me, because these sorts of Entertainments are principally design'd for the Ear and the Eye; and therefore in Reason my Art on this occasion, ought to be subservient to his."

The exact date of the premiere is unknown but the wordbook was advertised in The London Gazette from 4 to 8 June 1691, suggesting a recent staging. Peter Holman believes it was performed in May. The production was not as spectacular as Dioclesian or the later The Fairy Queen but it proved the most financially successful for the theatre. Betterton himself took the role of King Arthur, despite being in his fifties. The contemporary writer Roger North was most impressed by Charlotte Butler's singing of Cupid, describing it as "beyond anything I ever heard upon the stage", partly ascribing her success to "the liberty she had of concealing her face, which she could not endure should be so contorted as is necessary to sound well, before her gallants, or at least her envious sex."