The Red Book of Westmarch is a fictional manuscript written by hobbits, related to the author J. R. R. Tolkien's frame stories. It is an instance of the found manuscript conceit, a literary device to explain the source of his legendarium. In the fiction, it is a collection of writings in which the events of The Hobbit and The Lord of the Rings were recounted by their characters, and from which Tolkien supposedly derived these and other works. The name of the book comes from its red leather binding and casing, and from its having been housed in the Westmarch, a region of Middle-earth next to the Shire.

J. R. R. Tolkien used frame stories throughout his Middle-earth writings, especially his legendarium, to make the works resemble a genuine mythology written and edited by many hands over a long period of time. He described in detail how his fictional characters wrote their books and transmitted them to others, and showed how later in-universe editors annotated the material.