The Mousetrap is a murder mystery play by Agatha Christie. The longest-running West End show, it also has by far the longest run of any play in the world, reaching its 30,000th performance on 19 March 2025. The play opened in London's West End in 1952 and ran continuously until 16 March 2020, when the stage performances had to be temporarily discontinued during the COVID-19 pandemic. It then re-opened on 17 May 2021. As of 2022 the play had been seen by 10 million people in London.

A whodunit, the play has a twist ending which the audience are traditionally asked not to reveal after leaving the theatre. There are eight members of the cast, and by 2012 more than 400 actors had played the roles. Richard Attenborough was the original Detective Sergeant Trotter, and his wife, Sheila Sim, the first Mollie Ralston – owner of Monkswell Manor guesthouse. Since then few of the cast have been headliners, with Stephen Moss in The Guardian writing that "the play and its author are the stars".

The play began life as a short radio play written by Agatha Christie as a birthday present for Queen Mary, the consort of King George V. It was broadcast on 30 May 1947 under the name Three Blind Mice. The story drew from the real-life case of Dennis O'Neill, who died after he and his brother Terence suffered extreme abuse while in the foster care of a Shropshire farmer and his wife in 1945.

The play is based on a short story, itself based on the radio play, but Christie asked that the story not be published as long as it ran as a play in the West End of London. The short story has still not been published within the UK but it has appeared in the US in the 1950 collection Three Blind Mice and Other Stories.^{[citation needed]}

When she wrote the play, Christie gave the rights to her grandson Mathew Prichard as a birthday present. In the United Kingdom, only one production of the play in addition to the West End production can be performed annually, and under the contract terms of the play, no film adaptation can be produced until the West End production has been closed for at least six months. Producer John Woolf bought the film rights to the play in 1956, but Woolf and his heirs have been unable to produce an adaptation because the play has never closed. Woolf's son Jonathan hired Stuart Urban to write a screenplay for an adaptation of the play in the late 1990s, but the play's producers blocked Woolf from making the film. After unsuccessfully attempting to acquire the film rights to the play, Damian Jones was inspired to produce a film more loosely based on the play. Jones' idea became the 2022 film See How They Run, a murder mystery which takes place at the West End production of The Mousetrap in 1953, in which the murder victim is a director who has been hired to direct a film adaptation of the play.

The play had to be renamed at the insistence of Emile Littler, who had produced a play called Three Blind Mice in the West End before the Second World War. The suggestion to call it The Mousetrap came from Christie's son-in-law, Anthony Hicks.

The play's longevity has ensured its popularity with tourists from around the world. In 1997, at the initiative of producer Stephen Waley-Cohen, the theatrical education charity Mousetrap Theatre Projects was launched, helping young people experience London's theatre.

Tom Stoppard's 1968 play The Real Inspector Hound parodies many elements of The Mousetrap, including the surprise ending.