THX 1138 is a 1971 American social science fiction film co-written and directed by George Lucas in his directorial debut. Produced by Francis Ford Coppola and co-written by Walter Murch, the film stars Robert Duvall and Donald Pleasence, with Don Pedro Colley, Maggie McOmie, and Ian Wolfe in supporting roles. The film is set in a dystopian future in which the citizens are controlled by android police and mandatory use of drugs that suppress emotions.

THX 1138 was developed from Lucas's 1967 student film Electronic Labyrinth: THX 1138 4EB, which he created while attending the USC School of Cinematic Arts. The feature film was produced in a joint venture between Warner Bros. and American Zoetrope. A novelization by Ben Bova was published in 1971.

The film received mixed reviews from critics and underperformed at the box office upon its initial release, but it has subsequently received critical acclaim and gained a cult following, particularly in the aftermath of Lucas's success with Star Wars (1977). A director's cut prepared by Lucas was released in 2004.