Stranger on the Third Floor is a 1940 American film noir directed by Boris Ingster and starring Peter Lorre, John McGuire, Margaret Tallichet, and Charles Waldron, and featuring Elisha Cook Jr. It was written by Frank Partos. Modern research has shown that Nathanael West wrote the final version of the screenplay, but was uncredited.

Stranger on the Third Floor is often cited as the first "true" film noir of the classic period (1940–1959), though other films that fit the genre such as Rebecca and They Drive by Night were released earlier. Nonetheless, it has many of the hallmarks of film noir: an urban setting, heavy shadows, diagonal lines, voice-over narration, a dream sequence, low camera angles shooting up multi-story staircases, and an innocent protagonist desperate to clear himself after being falsely accused of a crime.