Irma la Douce (French: [iʁ.ma la dus], "Irma the Sweet") is a 1963 American romantic comedy film directed by Billy Wilder from a screenplay he co-wrote with I. A. L. Diamond, based on the 1956 French stage musical of the same name by Marguerite Monnot and Alexandre Breffort. The film stars Jack Lemmon and Shirley MacLaine.

Nestor Patou, an honest policeman, has been transferred from the Bois de Boulogne to Les Halles, a more urban neighborhood in Paris. He finds a street full of prostitutes working at the Hotel Casanova and raids the place, arresting the prostitutes and catching his police department's chief at the same time (while not knowing him). The inspector fires Nestor, who is mistakenly framed for bribery and for insubordination.

Kicked off the force and humiliated, Nestor finds himself drawn to the very neighborhood that ended his career with the Paris police—returning to Chez Moustache, a popular tavern for prostitutes and pimps. Down on his luck, Nestor befriends Irma la Douce, a popular prostitute. He reluctantly accepts, as a confidant, the proprietor of Chez Moustache, a man known only as Moustache. In a running joke, Moustache tells of a storied prior life, claiming to have been, among other things, an attorney, a colonel in the Foreign Legion, and a doctor who worked with Albert Schweitzer in Africa, ending with the repeated line, "but that's another story". After saving Irma from her abusive pimp, Hippolyte, Nestor moves in with her and unwittingly becomes a new one.

Nestor becomes infatuated and devises a plan to derail Irma's life as a prostitute. With the help of Moustache, Nestor disguises himself as Lord X, a wealthy English lord, who becomes Irma's exclusive client. Lord X has supposedly been rendered impotent by his service in World War II but is eager to support her in exchange for two visits each week. To finance Lord X's expensive habit, Nestor takes graveyard shifts in the marketplace; since he is missing all night and tired all day, Irma suspects an affair.

Irma seduces Lord X and persuades him to take her to England. At that point, Nestor decides to end the charade and kill off his alter ego. Unaware he is being tailed by Hippolyte, he tosses his disguise into the Seine. Seeing Lord X's clothes floating in the water, Hippolyte concludes Nestor killed him.

Arrested and sent to prison, Nestor escapes when he discovers that Irma is pregnant. He narrowly avoids being recaptured when the police search for him at the apartment; donning his old uniform, Nestor blends in with the other police and eludes capture.

With the help of Hippolyte, Nestor arranges for the police to search for him along the Seine, from which he emerges dressed as Lord X. Exonerated of the murder, Nestor and Irma agree to get married. At the church, Irma goes into labor and has their baby. Moustache identifies the real Lord X as a guest. As Lord X leaves, a clearly baffled Moustache looks at the audience.

The film was conceived in 1962 as a vehicle for Marilyn Monroe. The project would have reunited her with Wilder and Lemmon, both of whom had worked with her on Some Like It Hot. After Monroe's death, Shirley MacLaine was cast in the film after starring in The Apartment. MacLaine was paid $350,000 plus a percentage.