The 1983 psychological horror film Of Unknown Origin is the North American movie directing debut of filmmaker George P. Cosmatos. The cast includes Peter Weller, Jennifer Dale, Lawrence Dane, Maury Chaykin, Kenneth Welsh, and Louis Del Grande, along with Shannon Tweed in her film debut. Based on the 1979 novel The Visitor by Chauncey G. Parker III, Of Unknown Origin centers on a mild-mannered Manhattan banker who becomes increasingly obsessive and destructive in his attempts to kill a rat running loose in his newly-renovated brownstone.

Of Unknown Origin is co-production between Canada and the United States and was primarily shot in Montreal, with some additional shooting taking place in New York City. Legendary horror writer Stephen King has named it one of his favorite horror films.