Within the fiction of J.R.R. Tolkien, the Red Book of Westmarch (sometimes Red Book of the Periannath) is the book in which the events of The Hobbit and The Lord of the Rings were written. It is bound in red leather.

The Red Book was written by the Hobbit Bilbo Baggins and his heir Frodo Baggins, and contained both their adventures, as well as a lot of background information which the Bagginses collected. The Book was started by Bilbo Baggins, and recounted his quest for Erebor, which he called There and Back Again. He gave the Book to Frodo at Rivendell after completing it, and Frodo organized Bilbo's manuscript and used it to write down his own quest during the War of the Ring. Inscribed within, it reads:

Bilbo's translations of legends from the Elder Days were also added to it, as were various Hobbit poems and a lot of background information on the realms of Arnor, Gondor and Rohan, added to it by Peregrin Took and Meriadoc Brandybuck.

After Bilbo and Frodo left for Valinor, the Red Book passed into the keeping of Samwise Gamgee, mayor of the Shire. The book was left in the possession of Sam Gamgee's eldest daughter, Elanor Fairbairns, and her descendants (the Fairbairns of the Towers or Wardens of Westmarch). Several copies, with various notes and later additions, were made and the original was kept in a red case (with a three-volume Elvish Translation and a fifth volume [genealogical tables and commentaries]). Copies were passed on to future generations, of which one, the "Thain's Book", is the most important.

In the first edition of The Fellowship of the Ring, Tolkien's foreword claimed he had translated the Red Book from the original Westron into English, and it therefore must be supposed that copies of the book survived through several Ages.

The contents of the Red Book were probably as follows:

• Bilbo's journey: The Hobbit
• Frodo's journey: The Lord of the Rings
• Background information: the Appendices to The Lord of the Rings
• Hobbit poetry and legends: The Adventures of Tom Bombadil
• Bilbo's translation of legends: The Silmarillion

Much of the background information was added by Meriadoc Brandybuck and Peregrin Took, from their contacts in Rohan and Gondor. Other material was provided by King Elessar.

The "original" version of the Red Book contained the story of Bilbo's journey as it originally stood in the first edition of the Hobbit: thus, Gollum willingly gives the One Ring to Bilbo, and there is no trace of the Ring's hold over Gollum. Later copies of the Red Book contained, as an alternative, also the true account (later written in by Frodo), where Bilbo comes across the Ring by accident (the story as it stands in the current edition of the Hobbit and the Lord of the Rings).

In Peter Jackson's movie trilogy, the Red Book appears at the end of The Return of the King, where Frodo entrusts the book to Samwise just before he leaves Middle-earth. It also seen in the extended version of The Fellowship of the Ring, where Bilbo is seen writing in it at Rivendell.

Tolkien's inspiration for this repository of lore was the real Red Book of Hergest, the early 15th century compilation of Welsh history and poetry that contains the manuscript of the Mabinogion. Bound (and rebound) in red leather, in the Bodleian Library, Oxford, the manuscript was well known to Tolkien.

See also: Third Age, Westmarch