Unnatural Death, published in 1927, is the third novel written by Dorothy L. Sayers featuring her aristocratic detective Lord Peter Wimsey.

The story begins with a conversation in a restaurant between Wimsey, his friend Detective Inspector Charles Parker, and a doctor who tells them about a situation he was involved in: an elderly lady, suffering from a slow-acting cancer, died suddenly and unexpectedly with no obvious immediate cause of death. She died intestate, but her great-niece, with whom she was living, was set to inherit the considerable estate. Suspecting something wrong, the doctor demanded an autopsy, which showed nothing unusual, but stirred up such local animosity that the he was forced to abandon his practice. Wimsey, sensing a mystery, decides to investigate—but his investigation triggers a series of deadly events.

One of the delights of the book is the introduction of a new character in Miss Alexandra Climpson, a middle-aged spinster whom Wimsey employs as an investigative agent, and whose effusive reports of the gossip she picks up in the town are very amusing.

Unnatural Death is notable for its inclusion of one clearly lesbian character—a decision unusual in detective fiction at the time—and the very sympathetic treatment by Wimsey of a black character (though offensively racist terms for him are used by others in the book).

An adaptation of the book was made for BBC radio in 1975.