The Mystery of Mr. X is a 1934 American pre-Code crime film starring Robert Montgomery as a jewel thief who gets mixed up in a series of murders in London.

By chance, one of the murders occurs around the same time and place as a diamond robbery, leading Police Commissioner Sir Herbert Frensham (Henry Stephenson) to suspect the same man is responsible for both, much to the annoyance of the thief, Nicholas "Nick" Revel (Robert Montgomery), and his confederates, taxi driver Joseph "Joe" Palmer (Forrester Harvey) and insurance clerk Hutchinson (Ivan F. Simpson).

After another slaying, Sir Christopher Marche is arrested as a suspect, as he had drunkenly quarreled with the latest victim shortly before his death.

As a result, he becomes acquainted with Marche's grateful fiancée (and the commissioner's daughter), Jane Frensham (Elizabeth Allan).

After a struggle, Mr. X is fatally injured, but before he dies, he boasts to Sir Herbert how close he came to fulfilling his goal of one murder for each of the 15 years he spent in prison.