Secret Beyond the Door is a 1947 American film noir psychological thriller and a modern updating of the Bluebeard fairytale, directed by Fritz Lang, produced by Lang's Diana Productions, and released by Universal Pictures. The film stars Joan Bennett and was produced by her husband Walter Wanger. The black-and-white film noir drama is about a woman who suspects her new husband, an architect, plans to kill her.

"Bluebeard" is a French folktale, the most famous surviving version of which was written by Charles Perrault and first published by Barbin in Paris in 1697 in Histoires ou contes du temps passé. The tale tells the story of a wealthy man in the habit of murdering his wives and the attempts of the present one to avoid the fate of her predecessors. "The White Dove", "The Robber Bridegroom", and "Fitcher's Bird" are tales similar to "Bluebeard". The notoriety of the tale is such that Merriam-Webster gives the word Bluebeard the definition of "a man who marries and kills one wife after another". The verb bluebearding has even appeared as a way to describe the crime of either killing a series of women, or seducing and abandoning a series of women.