Who Is Number One? is a 1917 American silent mystery film serial directed by William Bertram and written by Anna Katharine Green. The film stars Kathleen Clifford, Cullen Landis, Gordon Sackville, Neil Hardin, Bruce Smith, and Ethel Ritchie. The film serial was released on October 29, 1917, by Paramount Pictures. It is presumed to be a lost film.