The Murderer Lives at Number 21 (French: L'Assassin habite au 21) is a 1942 French comedy thriller film by director Henri-Georges Clouzot. Adapted by Belgian writer Stanislas-André Steeman and Clouzot from Steeman's 1939 book of the same title, it was Clouzot's debut feature film as a director. The film is about the hunt by detective Wens (Pierre Fresnay) for the murderer Monsieur Durand, who leaves calling cards and manages to be everywhere at once. With the aspiring singer Mila Malou (Suzy Delair), Wens follows clues to a seedy boarding house where he hopes to find the murderer.

The Murderer Lives at Number 21 was the fourth film written by Clouzot for the Nazi run film company Continental Films who made films to take the place of banned American films. Clouzot made several changes from the script including the characters Mila and Wens from his previous screenplay for Le dernier des six (1941). The film was released in France to critical acclaim.