Persons Unknown is a 1929 mystery play by the British writer Edgar Wallace. The plot revolves around the murder of a "person unknown" in a street by a mysterious blackmailer.^[1] It features the character of Sergeant Elk, a Scotland Yard detective who appeared in several of Wallace's novels.

Its original run lasted for 108 performances, from 8 May 1929^[2] to 10 August 1929,^[3] at London's Shaftesbury Theatre. The cast included Gordon Harker and Minnie Rayner.