Armchair detectives are people who does not take an active part in the investigation of a crime, but attempts to solve it. They do not visit the crime scene, interview suspects, or have any first-hand knowledge of the crime. They rely on information from other sources, such as people and newspapers, to form their ideas.

Edgar Allen Poe's character Auguste Dupin is often thought to be the first 'armchair detective,' as he solves the disappearance of a woman by reading newspaper articles. The phrase is traced to the Sherlock Holmes story "The Greek Interpreter."