Middle-earth is the setting of much of the English writer J. R. R. Tolkien's fantasy. The term is equivalent to the Miðgarðr of Norse mythology and Middangeard in Old English works, including Beowulf. Middle-earth is the oecumene (i.e. the human-inhabited world, or the central continent of Earth) in Tolkien's imagined mythological past. Tolkien's most widely read works, The Hobbit and The Lord of the Rings, are set entirely in Middle-earth. "Middle-earth" has also become a short-hand term for Tolkien's legendarium, his large body of fantasy writings, and for the entirety of his fictional world.

Middle-earth is the main continent of Earth (Arda) in an imaginary period of the past, ending with Tolkien's Third Age, about 6,000 years ago. Tolkien's tales of Middle-earth mostly focus on the north-west of the continent. This region is suggestive of Europe, the north-west of the Old World, with the environs of the Shire reminiscent of England, but, more specifically, the West Midlands, with the town at its centre, Hobbiton, at the same latitude as Oxford.

Tolkien's Middle-earth is peopled not only by Men, but by Elves, Dwarves, Ents, and Hobbits, and by monsters including Dragons, Trolls, and Orcs. Through the imagined history, the peoples other than Men dwindle, leave or fade, until, after the period described in the books, only Men are left on the planet.

Tolkien's stories chronicle the struggle to control the world (called Arda) and the continent of Middle-earth between, on one side, the angelic Valar, the Elves and their allies among Men; and, on the other, the demonic Melkor or Morgoth (a Vala fallen into evil), his followers, and their subjects, mostly Orcs, Dragons and enslaved Men. In later ages, after Morgoth's defeat and expulsion from Arda, his place is taken by his lieutenant Sauron, a Maia.

The Valar withdrew from direct involvement in the affairs of Middle-earth after the defeat of Morgoth, but in later years they sent the wizards or Istari to help in the struggle against Sauron. The most important wizards were Gandalf the Grey and Saruman the White. Gandalf remained true to his mission and proved crucial in the fight against Sauron. Saruman, however, became corrupted and sought to establish himself as a rival to Sauron for absolute power in Middle-earth. Other races involved in the struggle against evil were Dwarves, Ents and most famously Hobbits. The early stages of the conflict are chronicled in The Silmarillion, while the final stages of the struggle to defeat Sauron are told in The Hobbit and in The Lord of the Rings.

Conflict over the possession and control of precious or magical objects is a recurring theme in the stories. The First Age is dominated by the doomed quest of the elf Fëanor and most of his Noldorin clan to recover three precious jewels called the Silmarils that Morgoth stole from them (hence the title The Silmarillion). The Second and Third Age are dominated by the forging of the Rings of Power, and the fate of the One Ring forged by Sauron, which gives its wearer the power to control or influence those wearing the other Rings of Power.

In ancient Germanic mythology, the world of Men is known by several names. The Old English middangeard descends from an earlier Germanic word and so has cognates such as the Old Norse Miðgarðr from Norse mythology, transliterated to modern English as Midgard. The original meaning of the second element, from proto-Germanic gardaz, was "enclosure", cognate with English "yard"; middangeard was assimilated by folk etymology to "middle earth". Middle-earth was at the centre of nine worlds in Norse mythology, and of three worlds (with heaven above, hell below) in some later Christian versions.

Tolkien's first encounter with the term middangeard, as he stated in a letter, was in an Old English fragment he studied in 1913–1914: