Her Majesty's Theatre is a West End theatre situated on Haymarket in the City of Westminster, London. The present building was designed by Charles J. Phipps and was constructed in 1897 for actor-manager Herbert Beerbohm Tree, who established the Royal Academy of Dramatic Art at the theatre. Andrew Lloyd Webber’s musical version of The Phantom of the Opera opened in 1986. Produced by Cameron Mackintosh, the original cast was led by Michael Crawford and Sarah Brightman. Today, The Phantom of the Opera, the timeless classic, is still playing to capacity, making it the longest run and most successful production in the theatre’s history.

In the early decades of the 20th century, Tree produced spectacular productions of Shakespeare and other classical works, and the theatre hosted premieres by major playwrights such as George Bernard Shaw, J. M. Synge, Noël Coward and J. B. Priestley. Since the First World War, the wide stage has made the theatre suitable for large-scale musical productions, and the theatre has accordingly specialised in hosting musicals. 

The theatre has been home to record-setting musical theatre runs, notably the First World War sensation Chu Chin Chow and the most recent production, Andrew Lloyd Webber's The Phantom of the Opera, which played continuously at Her Majesty's between 1986 and March 2020.

The theatre was established by architect and playwright John Vanbrugh, in 1705, as the Queen's Theatre. Legitimate drama unaccompanied by music was prohibited by law in all but the two London patent theatres, and so this theatre quickly became an opera house. Between 1711 and 1739, more than 25 operas by George Frideric Handel premiered here. Joseph Haydn’s series of concerts in London took place here in the 1790s. In the early 19th century, the theatre hosted the opera company that was to move to the Theatre Royal, Covent Garden, in 1847, and presented the first London performances of Mozart's La clemenza di Tito, Così fan Tutte and Don Giovanni. It also hosted the Ballet of Her Majesty's Theatre in the mid-19th century, before returning to hosting the London premieres of such operas as Bizet's Carmen and Wagner's Ring Cycle. The theatre has also been known as Queen's Theatre at the Haymarket, prior to being renamed Her Majesty's Theatre.

The theatre's capacity is 1,216 seats, and the building was Grade II* listed by English Heritage in 1970. LW Theatres has owned the building since 2000. The land beneath it is on a long-term lease from the Crown Estate. The name of the theatre changes with the gender of the monarch. Throughout the reign of Queen Victoria it was called Her Majesty's Theatre, changing to His Majesty's on the accession of Edward VII in 1901. In 1952 the theatre again became Her Majesty's on the accession of Elizabeth II. Following the accession of Charles III in 2022 the name reverted to His Majesty's.