The Sign of the Four -  is a novel by Arthur Conan Doyle, a British writer and medical doctor. He created the characters of Sherlock Holmes and Dr. Watson. The Sherlock Holmes stories are considered milestones in the field of crime fiction.

The Sign of the Four is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Set in 1888, The Sign of the Four has a complex plot involving service in India, the Indian Rebellion of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards. It presents Holmes's drug habit and humanizes him in a way that had not been done in the preceding novel. It also introduces Dr. Watson's future wife, Mary Morstan.