"The Adventure of the Second Stain", one of the 56 Sherlock Holmes short stories written by Sir Arthur Conan Doyle, is one of 13 stories in the cycle collected as The Return of Sherlock Holmes (1905) and the only unrecorded case mentioned passively by Watson to be written. It was first published in The Strand Magazine in the United Kingdom in December 1904, and was also published in Collier's in the United States on 28 January 1905. Doyle ranked "The Adventure of the Second Stain" eighth in his list of his twelve favourite Holmes stories.

Sherlock Holmes is a fictional detective created by British author Arthur Conan Doyle. Referring to himself as a "consulting detective" in his stories, Holmes is known for his proficiency with observation, deduction, forensic science and logical reasoning that borders on the fantastic, which he employs when investigating cases for a wide variety of clients, including Scotland Yard.