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 are 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, much of it inspired by French dance rhythms and adventurous (for the day) harmonies.