The Mysterious Monsieur Sylvain (French: Le mystérieux Monsieur Sylvain) is a 1947 French mystery film directed by Jean Stelli and starring Frank Villard, Simone Renant and Jean Chevrier. It was shot at the Billancourt Studios in Paris with sets designed by the art director Emile Alex.
When an engineer at a naval base is murdered three different men turn up claiming to be Monsieur Sylvain, a detective assigned to the case.