The Exorcist III is a 1990 American supernatural psychological horror film written for the screen and directed by William Peter Blatty, based on his 1983 novel Legion. It is the third installment in The Exorcist film series, and the final installment in Blatty's "Trilogy of Faith" after The Ninth Configuration (1980). The film stars George C. Scott, Ed Flanders, Jason Miller, Scott Wilson, Nicol Williamson, and Brad Dourif.

The Exorcist III ignores the events of Exorcist II: The Heretic (1977). The film follows a character from the original film, Lieutenant William F. Kinderman, who investigates a series of demonic murders in Georgetown that have the hallmarks of the Gemini, a deceased serial killer. Blatty based aspects of the Gemini Killer on the real-life Zodiac Killer, one of several serial killers who enjoyed the original The Exorcist.

Blatty, who wrote the 1971 novel The Exorcist and the screenplay for its 1973 film adaptation, conceived The Exorcist III with The Exorcist director William Friedkin attached to direct. When Friedkin left the project, Blatty adapted the script into the 1983 novel Legion. Morgan Creek Productions bought the film rights, with Blatty as director. Initially designed to be a standalone adaptation of that novel (hence the anachronistic elements which contradict Friedkin's adaptation of The Exorcist), the final cut wound up being an official entry in the filmed series due to Morgan Creek demanding extensive last-minute changes — including the insistence of bringing back a key cast member from the 1973 film, as well as an exorcism sequence for the climax — much to Blatty's frustration. Though some of the original footage appears permanently lost, Scream Factory released a director's cut closer to Blatty's vision in 2016, with footage assembled from various sources.

The Exorcist III was released in the United States on August 17, 1990, by 20th Century Fox as part of their multi-picture deal with Morgan Creek that commenced in 1987 and concluded in 1991, when the latter switched distribution through Warner Bros. Pictures. As such, this was the only film at that point in the series which was not distributed by Warner Bros. until The Exorcist: Believer was released by Universal Pictures in 2023. The film received mixed reviews from critics and grossed $44 million domestically against a production budget of $11 million.