The Silmarillion (Quenya: [silmaˈrilːiɔn]) is a book consisting of a collection of myths and stories in varying styles by the English writer J. R. R. Tolkien. It was edited, partly written, and published posthumously by his son Christopher in 1977, assisted by Guy Gavriel Kay, who became a fantasy author. It tells of Eä, a fictional universe that includes the Blessed Realm of Valinor, the ill-fated region of Beleriand, the island of Númenor, and the continent of Middle-earth, where Tolkien's most popular works—The Hobbit and The Lord of the Rings—are set. After the success of The Hobbit, Tolkien's publisher, Stanley Unwin, requested a sequel, and Tolkien offered a draft of the writings that would later become The Silmarillion. Unwin rejected this proposal, calling the draft obscure and "too Celtic", so Tolkien began working on a new story that eventually became The Lord of the Rings.

The Silmarillion has five parts. The first, Ainulindalë, tells in mythic style of the creation of Eä, the "world that is." The second part, Valaquenta, gives a description of the Valar and Maiar, supernatural powers of Eä. The next section, Quenta Silmarillion, which forms the bulk of the collection, chronicles the history of the events before and during the First Age, including the wars over three jewels, the Silmarils, that gave the book its title. The fourth part, Akallabêth, relates the history of the Downfall of Númenor and its people, which takes place in the Second Age. The final part, Of the Rings of Power and the Third Age, tells the history of the rings during the Second and Third Ages, ending with a summary of the events of The Lord of the Rings.

The book shows the influence of many sources, including the Finnish epic Kalevala, as well as from Greek mythology, including the lost island of Atlantis (as Númenor) and the Olympian gods (in the shape of the Valar, though these also resemble the Norse Æsir).

Because J. R. R. Tolkien died leaving his legendarium unedited, Christopher Tolkien selected and edited materials to tell the story from start to end. In a few cases, this meant that he had to devise completely new material, within the tenor of his father's thought, to resolve gaps and inconsistencies in the narrative, particularly Chapter 22, "Of the Ruin of Doriath".

The Silmarillion was commercially successful, but received generally poor reviews on publication. Scholars found the work problematic, not least because the book is a construction, not authorised by Tolkien himself, from the large corpus of documents and drafts also called "The Silmarillion". Scholars have noted that Tolkien intended the work to be a mythology, penned by many hands, and redacted by a fictional editor, whether Ælfwine or Bilbo Baggins. As such, Gergely Nagy considers that the fact that the work has indeed been edited actually realises Tolkien's intention.

The events described in The Silmarillion, as in J. R. R. Tolkien's extensive Middle-earth writings which the book summarises, were meant to have taken place at some time in Earth's past. In keeping with this idea, The Silmarillion was supposedly translated from Bilbo's three-volume Translations from the Elvish, which he wrote while at Rivendell. The book covers the history of the world, Arda, up to the Third Age, in its five sections:

Ainulindalë (Quenya: "The Music of the Ainur") takes the form of a primary creation narrative. Eru ("The One"), also called Ilúvatar ("Father of All"), first created the Ainur, a group of eternal spirits or demiurges, called "the offspring of his thought". Ilúvatar brought the Ainur together and showed them a theme, from which he bade them make a great music. Melkor—whom Ilúvatar had given the "greatest power and knowledge" of all the Ainur—broke from the harmony of the music to develop his own song. Some Ainur joined him, while others continued to follow Ilúvatar, causing discord in the music. This happened three times, with Eru Ilúvatar successfully overpowering his rebellious subordinate with a new theme each time. Ilúvatar then stopped the music and showed them a vision of Arda and its peoples. The vision disappeared, and Ilúvatar offered the Ainur the opportunity to enter into Arda and govern the new world.

Many Ainur accepted, taking physical form and becoming bound to that world. The greater Ainur became the Valar, while the lesser Ainur became the Maiar. The Valar attempted to prepare the world for the coming inhabitants (Elves and Men), while Melkor, who wanted Arda for himself, repeatedly destroyed their work; this went on for thousands of years and, through waves of destruction and creation, the world took shape.