M is a 1931 German mystery thriller film directed by Fritz Lang and starring Peter Lorre as Hans Beckert, a serial killer who targets children, in his third screen role. Both Lang's first sound film and an early example of a procedural drama, M centers on the efforts of both a city's police force and its criminal syndicates to apprehend a serial child-murderer.

The film's screenplay was written by Lang and his wife Thea von Harbou. It features many cinematic innovations, including the use of long tracking shots and a musical leitmotif in the form of "In the Hall of the Mountain King", which is repeatedly whistled by Lorre's character. Lang regarded the film as his magnum opus, and it is widely considered one of the greatest films of all time and an indispensable influence on modern crime and thriller fiction.

An American remake under the same title, directed by Joseph Losey, was released in 1951.