Sherlock Holmes and the Secret Weapon (1942) is the fourth in the Basil Rathbone/Nigel Bruce series of 14 Sherlock Holmes films which updated the characters created by Sir Arthur Conan Doyle to the then present day. The film is credited as an adaptation of Conan Doyle's 1903 short story "The Adventure of the Dancing Men," though the only element from the source material is the dancing men code. Rather, it is a spy film taking place on the background of the then ongoing Second World War with an original premise. The film concerns the kidnapping of a Swiss scientist by their nemesis Professor Moriarty, to steal a new bomb sight and sell it to Nazi . Sherlock Holmes and Dr. John Watson have to crack a secret code in order to save the country.

The film is one of four films in the series which are in the public domain.