The Crystal Stopper is a mystery novel by Maurice Leblanc featuring the adventures of the gentleman thief Arsène Lupin. Maurice Leblanc was inspired by the infamous Panama scandals of 1892 and 1893. The novel borrows from Edgar Allan Poe's short story The Purloined Letter the idea of hiding an object in plain sight.It mainly tells that during a burglary at the home of Deputy Daubrecq a crime is committed and two accomplices of Arsène Lupin are arrested by the police. One is guilty of the crime, the other innocent but both will be sentenced to death. Lupin seeks to deliver the victim of a miscarriage of justice, but struggles against Deputy Daubrecq's ruthless blackmailer, who has an incriminating document hidden in a crystal stopper.

Maurice Marie Émile Leblanc (11 November 1864 – 6 November 1941) was a French novelist and writer of short stories, known primarily as the creator of the fictional gentleman thief and detective Arsène Lupin, often described as a French counterpart to Arthur Conan Doyle's creation Sherlock Holmes.