Dioclesian (The Prophetess: or, The History of Dioclesian) is an English tragicomic semi-opera in five acts by Henry Purcell to a libretto by Thomas Betterton based on the play The Prophetess, by John Fletcher and Philip Massinger, which in turn was based very loosely on the life of the Emperor Diocletian.

Betterton reworked the play extensively,[1] making room for a great deal of Purcell's music, notably in the 'monster' scene at the end of Act II and the final Masque about the victory of Love, which remained popular until well into the eighteenth century.

The premier production had a Prologue written by John Dryden that was suppressed after only one performance; it was far too critical of King William's military campaign in Ireland.

She then causes the princess to fall in love with Diocles' rival Maximinian, and the Persians to defeat the Roman army.

Diocles realises the error of his ways, routs the invaders, cedes his half of the throne to Maximinian, and moves to Lombardy with Drusilla.