The Fortieth Door is a 1924 American adventure film serial directed by George B. Seitz and starring Allene Ray and Bruce Gordon. The film is considered to be lost. The Library of Congress includes the film among the National Film Preservation Board's updated 2019 list of "7,200 Lost U.S. Silent Feature Films" produced between 1912 and 1929.

George Brackett Seitz was an American playwright, screenwriter, film actor and director. He was known for his screenplays for action serials, such as The Perils of Pauline (1914) and The Exploits of Elaine (1914).