The Secret of Annexe 3 is a crime novel by Colin Dexter, the seventh novel in Inspector Morse series.

As the novel begins, Margaret Bowman of Charlbury Drive Chipping Norton is off to a funeral. Her husband, left alone, finds an angry letter, apparently from a lover, in his wife's handbag.

The guests of Haworth Hotel rise late on New Year's Day, with one exception, the guest in Annexe 3 who missed New Year's Day completely. He lies dead in his room on the blood-soaked bed.

After the murder, Inspector Morse, with the help of the receptionist Miss Sarah Jonstone, examines the letters and phone messages booking the various rooms at the hotel. Discovering the non-existent address, he deduces that a postman must be involved.

Thomas Bowman, the postman, turns out to be the corpse, and his wife and her lover are the instigators of the murder. Winston Grant, a black musician, was hired to provide the alibi.

Although the narratives differ, this novel provided the inspiration for the Inspector Morse television episode The Secret of Bay 5B, the last episode of series 3, aired in 1989.

• 1986, London: Macmillan ISBN 0-333-43139-1, Pub date October 1986, Hardback
• 1987, New York: St. Martin's Press ISBN 0-312-01089-3, Pub date November 1987, Hardback