The Theatre Royal Haymarket (also known as Haymarket Theatre or the Little Theatre) is a West End theatre on Haymarket in the City of Westminster that dates back to 1720, making it the third-oldest London playhouse still in use. In February 2019, Only Fools and Horses The Musical premiered at the theatre. It was announced that the production will close on 29 April 2023 following over 1000 performances marking it the longest-running production in the Theatre Royal Haymarket's history.

Samuel Foote acquired the lease in 1747, and in 1766 he gained a royal patent to play legitimate drama (meaning spoken drama, as opposed to opera, concerts or plays with music) in the summer months. The original building was a little further north in the same street. It has been at its current location since 1821, when it was redesigned by John Nash. It is a Grade I listed building, with a seating capacity of 888. The freehold of the theatre is owned by the Crown Estate.

The Haymarket has been the site of a significant innovation in theatre. In 1873, it was the venue for the first scheduled matinée performance, establishing a custom soon followed in theatres everywhere. Its managers have included Benjamin Nottingham Webster, John Baldwin Buckstone, Squire Bancroft, Cyril Maude, Herbert Beerbohm Tree, and John Sleeper Clarke, brother-in-law of John Wilkes Booth, who quit America after the assassination of Abraham Lincoln. Famous actors who débuted at the theatre included Robert William Elliston (1774–1831) and John Liston (1776–1846).

Productions at the Haymarket in this century have included The Royal Family by Edna Ferber, starring Judi Dench (2001), Lady Windermere's Fan, directed by Peter Hall, starring Vanessa Redgrave and Joely Richardson (2002), and Dench appeared on stage together with Maggie Smith for the first time in over 40 years in The Breath of Life by David Hare (2002). Productions in 2003 included Ibsen's Brand, directed by Adrian Noble, starring Ralph Fiennes and A Woman of No Importance, with Rupert Graves, Samantha Bond and Prunella Scales, also directed by Noble. In 2004, the theatre presented a stage adaptation of the film, When Harry Met Sally..., starring Luke Perry and Alyson Hannigan, during which the house closed for two nights after bits of the ceiling fell during a performance injuring fifteen people.

2005 productions included Victoria Wood's Acorn Antiques The Musical, starring Julie Walters, Celia Imrie and Duncan Preston, directed by Trevor Nunn and A Few Good Men, starring Rob Lowe, Suranne Jones and Jack Ellis. 2006 featured three revivals: A Man for All Seasons, starring Martin Shaw; Coward's Hay Fever, with Judi Dench and Peter Bowles; and Seven Brides for Seven Brothers, starring Dave Willetts and Shona Lindsay. The last production of that year was Six Dance Lessons in Six Weeks, starring Claire Bloom and Billy Zane. The first production of 2007 was Pinter's People, a compilation of Harold Pinter sketches of the past 40 years; later productions of that year were The Lady from Dubuque (Albee), starring Maggie Smith; David Suchet in The Last Confession; and The Country Wife, starring Toby Stephens, Patricia Hodge and David Haig.

In 2008, productions were The Sea (Bond), starring David Haig, Eileen Atkins and Russell Tovey; Marguerite, a new musical starring Ruthie Henshall and Alexander Hanson; and Keith Allen in an adaptation of Treasure Island. The following year, Ian McKellen, Patrick Stewart, Simon Callow and Ronald Pickup starred in Waiting for Godot, followed by Breakfast at Tiffany's, starring Anna Friel, Joseph Cross, James Dreyfus and Suzanne Bertish. Godot and Tiffany's were featured, along with the staff and history of the Haymarket Theatre itself, in a 2009 eight-part Sky Arts documentary, Theatreland. In 2010 Waiting for Godot was repeated with McKellen, Roger Rees, Matthew Kelly and Pickup, followed by a transfer of Sweet Charity from the Menier Chocolate Factory. The next show was The Rivals starring Penelope Keith and Peter Bowles.

Trevor Nunn became Artistic Director 2011, producing a revival of Flare Path, as part of the playwright Terence Rattigan's centenary year celebrations, starring Sienna Miller, James Purefoy and Sheridan Smith; the Chichester Festival Theatre's revival of Rosencrantz and Guildenstern are Dead by Tom Stoppard; Ralph Fiennes as Prospero in The Tempest; and, over the Christmas/New Year season, Robert Lindsay and Joanna Lumley in The Lion in Winter. For two years from March 2012, the Haymarket hosted the National Theatre production One Man, Two Guvnors, which transferred from the Adelphi Theatre. The theatre was one of the 40 theatres featured in the 2012 DVD documentary series Great West End Theatres, presented by Donald Sinden.

In 2014, a stage adaptation of the film Fatal Attraction, directed by Nunn, premiered at the theatre, and Maureen Lipman and Harry Shearer starred in Daytona. The following year Penelope Wilton starred in Taken At Midnight. This was followed by Harvey, starring James Dreyfus and Maureen Lipman, and The Elephant Man, starring Bradley Cooper. McQueen, starring Stephen Wight, then transferred from the St. James Theatre, and was followed by Mr Foote's Other Leg, starring Simon Russell Beale as Samuel Foote.

Productions in 2016 included a revival of Alan Ayckbourn's How the Other Half Loves, starring Nicholas Le Prevost, Jenny Seagrove, Tamzin Outhwaite and Jason Merrells, and Pixie Lott made her debut at the Haymarket as Holly Golightly in Breakfast at Tiffany's. In December the Royal Shakespeare Company took up residence at the Haymarket with a double bill of Love's Labour's Lost and Much Ado About Nothing. In 2017, Damian Lewis and Sophie Okonedo starred in Edward Albee's The Goat, or Who is Sylvia? from March to June. The RSC then returned to the theatre with Queen Anne. Natalie Dormer and David Oakes later starred in Venus in Fur.

In 2018, Suranne Jones, Jason Watkins and Nina Sosanya starred in a revival of Frozen, a play by Bryony Lavery, followed by Heathers: The Musical starring Carrie Hope Fletcher. In February 2019, Only Fools and Horses The Musical premiered at the theatre.