A mockbuster, also known as knockbuster, is a film created to exploit the publicity of another major motion picture with a similar title or subject. Mockbusters are often made with a low budget and quick production to maximize profits. The term is a portmanteau of the words "mock" or "knock" and "blockbuster".

Unlike films produced to capitalize on the popularity of a recent release by adopting similar genres or storytelling elements, mockbusters are generally produced concurrently with upcoming films and released direct-to-video around the time the film they are inspired by is released. A mockbuster may be similar enough in title or packaging that consumers confuse it with the actual film it mimics. These attempts are notorious for their plagiarism which have resulted in successful infringement lawsuits. However, their producers maintain that they are simply offering additional products for consumers who want to watch more films in the same subgenres.

Mockbusters have a long history in Hollywood and elsewhere. For example, the 1959 Vanwick film The Monster of Piedras Blancas was a clear derivative of Creature from the Black Lagoon, complete with a creature suit by the same designer, Jack Kevan. Attack of the 50 Foot Woman spawned Village of the Giants, and The Land That Time Forgot spawned Legend of Dinosaurs & Monster Birds.

Such films fit the B movie model, being produced on a small budget and derivative of the target film and other similar projects. The lower costs of using modern video and computer graphics equipment and the tie-in to the mainstream film's advertising have allowed the mockbuster to become a profitable niche in the home video market. Blockbuster, once the largest DVD rental chain, implied support to the concept by buying 100,000 copies of The Asylum's version of War of the Worlds during the theatrical opening week of Steven Spielberg's film based on the same novel starring Tom Cruise.

Most mockbusters capitalize on the popularity of theatrically released films, but some are derivative of a TV series. The 1979 film Angels Revenge bore many superficial similarities to the popular TV series Charlie's Angels; its promotional materials even resembled Charlie's Angels' graphic style. In reverse, Glen A. Larson was accused of producing mockbusters at the height of his career, with his television series plagiarizing popular films of the time (Battlestar Galactica, for example, capitalized on the popularity of Star Wars, while Alias Smith and Jones was a take on Butch Cassidy and the Sundance Kid).

In blaxploitation filmmaking, it was a common practice to title blaxploitation films after previously successful films starring predominantly white casts, and produce similarly titled films starring predominantly African American casts, as observed in the films Black Shampoo (1976, after Shampoo), Black Lolita (1975, after Lolita) and The Black Godfather (1974, after The Godfather). This would arguably also include blaxploitation renditions of classic horror stories, such as Blacula (1974) and Dr. Black, Mr. Hyde (1976).

GoodTimes Entertainment was notorious for distributing animated "mockbuster" counterparts to popular Disney films in the 1990s (such as those made by Golden Films); because Disney was creating its films based on public domain folk tales and historical stories, GoodTimes' actions were legal and survived Disney's legal challenge against it.

The company would eventually become bankrupt and in September 2005, was absorbed into Galam (now known as Gaia Inc.).