The undistributed middle is a logical fallacy of deduction in which the middle term of a syllogism is not distributed in at least one of the premises.

According to the rules of logic, a term is "distributed" when a sentence says something about everything the term designates. A syllogism is invalid if both middle terms are undistributed.

British educator Madsen Pirie illustrates the fallacy of the undistributed middle with this "schoolboy" argument: "because all horses have four legs and all dogs have four legs, so all horses are dogs."

"Both horses and dogs are indeed four-legged," Pirie points out, "but neither of them occupies the whole of the class of four-legged beings. This leaves convenient room for horses and dogs to be different from each other, and from other beings which might also without any overlap be in the four-legged class" (How to Win Every Argument: The Use and Abuse of Logic, 2007).