The Sondheim Theatre (formerly the Queen's Theatre) is a West End theatre located in Shaftesbury Avenue on the corner of Wardour Street in the City of Westminster, London. Cameron Mackintosh's production of the musical Les Miserables is taking place at the Sondheim Theater. The musical celebrated its 20th anniversary at the venue on 8 October 2005 and overtook Cats as the longest-running musical of all time. Set in early 19th-century France, Les Misérables is the story of Jean Valjean, a French peasant, and his desire for redemption, released in 1815 after serving nineteen years in jail for stealing a loaf of bread for his sister's starving child.

It opened as the Queen's Theatre on 8 October 1907, as a twin to the neighboring Hicks Theatre (now the Gielgud Theatre) which had opened ten months earlier. Both theatres were designed by W. G. R. Sprague. In 2019 the theatre's name was changed from the Queen's to the Sondheim Theatre. The theatre reopened on 18 December 2019.

The original plan was to name the venue the Central Theatre. However, after a lengthy debate, it was named the Queen's Theatre, and a portrait of Queen Alexandra was hung in the foyer.

The first production at the Queen's Theatre was a comedy by Madeleine Lucette Ryley called The Sugar Bowl. Although it was poorly received and ran for only 36 performances, the theatre received glowing reviews.

In September 1940, a German bomb landed directly on the theatre, destroying the facade and lobby areas. The production at the time was Daphne du Maurier's Rebecca starring Celia Johnson, Owen Nares, and Margaret Rutherford. The theatre remained closed until a £250,000 restoration was completed by Westwood Sons & Partners almost 20 years later. The auditorium retained its Edwardian decor, while the lobbies and exterior were rebuilt in a modern style. The reconstructed theatre opened on 8 July 1959 with John Gielgud's solo performance in Shakespeare speeches and sonnets, Ages of Man.

From April 2004 to July 2019, the theatre played host to Cameron Mackintosh's production of Les Misérables which transferred after 18 years at the nearby Palace Theatre. The musical celebrated its 20th anniversary at the venue on 8 October 2005 and overtook Cats as the longest-running musical of all time a year later on 8 October 2006.

The theatre was Grade II listed by English Heritage in June 1972.

An extensive refurbishment was undertaken in the latter half of 2009 which improved public areas and increased capacity with new seating and boxes reinstated at dress circle level.

In 2019, Cameron Mackintosh announced that the original production of Les Misérables would close on 13 July 2019 while the theatre underwent a major restoration, but would return in a new production from 18 December 2019, when the Queen's Theatre would be renamed as the Sondheim Theatre in honor of composer and lyricist Stephen Sondheim.