A Wolf in sheep's clothing is an idiom of Biblical origin used to describe those playing a role contrary to their real character with whom contact is dangerous, particularly false teachers. As a fable it has been falsely credited to Aesop and the theme is now numbered 451 in the Perry Index. The confusion has arisen from the similarity of the theme with fables of Aesop concerning wolves that are mistakenly trusted by shepherds; the moral drawn from these is that one's basic nature eventually betrays itself.

In the Dungeons & Dragons fantasy role-playing game, the wolf-in-sheep's-clothing is a type of fictional monster.