A legendarium is a book or series of books consisting of a collection of legends.

The word was also used by J. R. R. Tolkien to describe his artificial mythology of Middle-earth and Valinor. Tolkien's great mythological tales of Middle-earth are meant to be taken, fictitiously, as an ancient history of the Earth, particularly of Europe, from several thousand years before the lands took their present shape. Tolkien's legendarium is often called "Middle-earth mythology", but, strictly speaking, Middle-earth is only one continent in Tolkien's world of Arda.