Gas Light is a 1938 thriller play, set in 1880s London, written by the British novelist and playwright Patrick Hamilton. Hamilton's play is a dark tale of a marriage based on deceit and trickery, and a husband committed to driving his wife insane in order to steal from her.

Gas Light was written during a dark period in Hamilton's life. Six years prior to the play Hamilton was hit by a drunk driver and dragged through the streets of London, leaving him with a limp, a paralysed arm, and a disfigured face. Two years later, Hamilton's mother took her own life.

Premiering at the Richmond Theatre in London on 5 December 1938 before transferring to the Apollo Theatre in the West End on 1 January, the play closed after six months and 141 performances, but it has endured through an impressive list of incarnations most notably Five Chelsea Lane (1941 American play – renamed for Los Angeles production), Angel Street (1941 American play – renamed again when Los Angeles production transferred to Broadway), and Gaslight (1958 Australian television play). Angel Street was a hit in its Broadway premiere, and it remains one of the longest-running non-musicals in Broadway history, with 1,295 total performances.

The play was adapted to the big screen as two films, both entitled Gaslight—a 1940 British film, and a 1944 American film directed by George Cukor, also known as The Murder in Thornton Square in the UK. Both films are considered classics in their respective countries of origin, and are generally equally critically acclaimed. The 1944 American version received seven nominations at the 17th Academy Awards, including Best Picture, and won two, Best Actress (for Ingrid Bergman) and Best Production Design. In 2019, the film was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".