The Sign of Four (SIGN) is a Sherlock Holmes novel written by Arthur Conan Doyle first published in the Lippincott's Magazine in February 1890. This is the 2nd Sherlock Holmes story.

In the 7 and 14 June 1890 issues of The Bristol Observer appeared the first illustrations of Sherlock Holmes wearing a deerstalker, before the well-known Sidney Paget ones.