Of Unknown Origin is a 1983 psychological horror film directed by George P. Cosmatos in his North American film debut, and starring Peter Weller, Jennifer Dale, Lawrence Dane, Maury Chaykin, Kenneth Welsh, Louis Del Grande and Shannon Tweed in her film debut. Based on the 1979 novel The Visitor by Chauncey G. Parker III, it focuses on a mild-mannered Manhattan banker who becomes increasingly obsessive and destructive in his attempts to kill a rat loose in his renovated brownstone. The film's title refers to the misconception (repeated in the film) that rats have no known origin.

A co-production between Canada and the United States, the film was primarily shot in Montreal, with some additional shooting taking place in New York City. The film was released theatrically on November 24, 1983 by Warner Bros. It received generally positive reviews from critics, with much praise for Weller's leading performance. Writer Stephen King has named it one of his favorite horror films.