Murder by Decree is a 1979 mystery thriller film directed by Bob Clark. It features the Sherlock Holmes and Dr. John Watson characters created by Sir Arthur Conan Doyle, who are embroiled in the investigation surrounding the real-life 1888 Whitechapel murders committed by "Jack the Ripper". Christopher Plummer plays Holmes and James Mason plays Watson. Though it features a similar premise, it is somewhat different in tone and result to A Study in Terror. It is loosely based on The Ripper File by Elwyn Jones and John Lloyd.

The film's premise of the plot behind the murders is influenced by the book Jack the Ripper: The Final Solution (1976), by Stephen Knight, who theorised that the killings were part of a Masonic plot. The original script contained the names of the historical suspects, Sir William Gull and John Netley. In the actual film, they are represented by fictional analogues: Thomas Spivy (Gull) and William Slade (Netley). This plot device was later used in other Jack the Ripper-themed fiction, including the graphic novel From Hell.