"The Beast Within" was written by Tom Holland, in his first writing credit for a feature film, and directed by Philippe Mora. The film itself is very loosely based on the 1981 horror novel of the same name written by Edward Levy. Producer Harvey Bernhard had purchased the rights to Levy's then-unfinished novel based on the title alone. However, when pre-production for the film began, Levy had still not completed the novel due to marital difficulties. As a result, the film's writer Holland was forced to write most of the film's screenplay from scratch, with the final product differing significantly from the novel.