American romantic comedy film directed and produced by Billy Wilder from a screenplay he co-wrote with I. A. L. Diamond. It stars Jack Lemmon, Shirley MacLaine, Fred MacMurray, Ray Walston, Jack Kruschen, David Lewis, Willard Waterman, David White, Hope Holiday and Edie Adams.

The film follows an insurance clerk (Lemmon) who, in the hope of climbing the corporate ladder, lets more senior coworkers use his Upper West Side apartment to conduct extramarital affairs. He is attracted to an elevator operator (MacLaine) in his office building. He does not know she is having an affair with his immediate boss (MacMurray).

In the years since its release, The Apartment has come to be regarded as one of the greatest films ever made, appearing in lists by the American Film Institute and Sight and Sound magazine. In 1994, it was one of the 25 films selected for inclusion to the United States Library of Congress National Film Registry.

Although Wilder generally required his actors to adhere exactly to the script, he allowed Lemmon to improvise in two scenes: In one scene, he squirts a bottle of nasal spray across the room, and in another, he sings while cooking spaghetti (which he strains through the grid of a tennis racket). In another scene, where Lemmon was supposed to mime being punched, he failed to move correctly, and was accidentally knocked down. Wilder chose to use the shot of the genuine punch in the film. Lemmon also caught a cold when one scene on a park bench was filmed in sub-zero weather.