The Third Man is a 1949 British film noir directed by Carol Reed, written by Graham Greene, and starring Joseph Cotten, Alida Valli, Orson Welles and Trevor Howard. Set in post-World War II Allied-occupied Vienna, the film centres on American writer Holly Martins (Cotten), who arrives in the city to accept a job with his friend Harry Lime (Welles), only to learn that he has died. Martins stays in Vienna to investigate Lime's death, becoming infatuated with Lime's girlfriend Anna Schmidt (Valli).

The use of black-and-white German expressionist-influenced cinematography by Robert Krasker, with its harsh lighting and Dutch angles, is a major feature of The Third Man. Combined with the use of ruined locations in Vienna, the style evokes exhaustion and cynicism at the start of the Cold War.

Greene wrote a novella as a treatment for the screenplay. Composer Anton Karas' title composition "The Third Man Theme" topped the international music charts in 1950, bringing international fame to the previously unknown performer. The Third Man is considered one of the greatest films of all time, celebrated for its acting, musical score, and atmospheric cinematography.

In 1999, the British Film Institute voted The Third Man the greatest British film of all time. In 2011, a poll for Time Out ranked it the second-best British film ever.