Professor James Moriarty is a fictional character who is the arch-enemy of the detective Sherlock Holmes. Moriarty is a criminal mastermind described by Holmes as the "Napoleon of Crime". (T. S. Eliot would later the same phrase, in homage, to describe Macavity in Old Possum's Book of Practical Cats.)

Professor Moriarty first appeared in Arthur Conan Doyle's tale "The Final Problem", in which Holmes, on the verge of delivering a fatal blow to Moriarty's criminal organisation, is forced to flee to the Continent to escape Moriarty's retribution. Moriarty follows, and the two apparently fall to their deaths whilst locked in mortal combat atop the Reichenbach Falls.

Moriarty only plays a role in one other of Conan Doyle's Holmes stories: The Valley of Fear, published after but set before "The Final Problem", in which Holmes attempts to prevent a murder being carried out by Moriarty's agents. Moriarty himself does not appear, although he does send Holmes a note of commiseration at the end.

In addition to these, Holmes mentions Moriarty reminiscently in five other stories: "The Empty House", "The Norwood Builder", "The Missing Three-Quarter", "The Illustrious Client", and "His Last Bow".

Although Moriarty only appeared in two of the sixty Sherlock Holmes tales by Conan Doyle, Holmes's attitude to him in those two stories has gained him the popular impression of being Holmes' nemesis, and he has been frequently used in later stories by other authors, parodies, and in other media.

In the Conan Doyle stories, narrated by Holmes's assistant Dr. Watson, Moriarty is never seen by Watson, who relies upon Holmes to relate accounts of the detective's battle with the criminal. In stories by other writers, Watson has encountered Moriarty more often.

His weapon of choice was the "air-rifle", a unique weapon constructed for the Professor by a blind German mechanic, Von Herder, and used by his employee Colonel Sebastian Moran.

Holmes described Moriarty as being

a man of good birth and excellent education. endowed by nature with a phenomenal mathematical faculty. At the age of twenty-one he wrote a treatise upon the binomial theorem, which has had a European vogue. On the strength of it he won the mathematical chair at one of our smaller universities, and had, to all appearances, a most brilliant career before him.

The stories give a number of indications about the Professor's family, some seemingly contradictory. In The Valley of Fear, Holmes says of him: "He is unmarried. His younger brother is a station master in the west of England." In "The Final Problem", Watson refers to "the recent letters in which Colonel James Moriarty defends the memory of his brother". In neither story are we told the Professor's own first name; it is only in "The Empty House" that Holmes refers to Professor James Moriarty. The question of how many Moriarty brothers this makes, and which of them is called James, has provided much amusement for Sherlock Holmes fans in the years since the stories were first published.