He Knows You're Alone is a 1980 American psychological slasher film directed by Armand Mastroianni, written by Scott Parker and starring Caitlin O'Heaney, Don Scardino, Elizabeth Kemp, Tom Rolfing, and Tom Hanks in his film debut. The plot follows a woman who is stalked by a killer targeting soon-to-be brides the weekend before her wedding.

Independently made by several producers, including Edgar Lansbury, He Knows You're Alone was shot on location in Mastroianni's native Staten Island, New York in December 1979 under the working title Blood Wedding. The film was subsequently sold to Metro-Goldwyn Mayer, who retitled it and released it with United Artists on August 29, 1980. Although the film received mixed reviews, it was a commercial success for MGM, grossing nearly $5 million at the U.S. box office.

He Knows You're Alone has been credited for being one of the first horror films inspired by the success of Halloween, and shares a number of similarities with that previous hit. In the years since its release, it has received some retrospective praise from genre film critics. Film critic Robin Wood noted the film among its peers as a "highly sophisticated attempt" at analyzing violence against women.