In J. R. R. Tolkien's fictional continent of Middle-earth, Mordor (pronounced [ˈmɔrdɔr]; from Sindarin Black Land and Quenya Land of Shadow) is a dark realm. It lay to the east of Gondor and the great river Anduin, and to the south of Mirkwood. Mount Doom, a volcano in Mordor, was the goal of the Fellowship of the Ring in the quest to destroy the One Ring. Mordor was surrounded by three mountain ranges, to the north, the west, and the south. These both protected the land from invasion and kept those living in Mordor from escaping.

Commentators have noted that Mordor was influenced by Tolkien's own experiences in the industrial Black Country of the English Midlands, and by his time fighting in the trenches of the Western Front in the First World War. Tolkien was also familiar with the account of the monster Grendel's unearthly landscapes in the Old English poem Beowulf. Others have observed that Tolkien depicts Mordor as specifically evil, and as a vision of industrial environmental degradation, contrasted with either the homey Shire or the beautiful elvish forest of Lothlórien.

Mordor was roughly rectangular in shape, with the longer sides on the north and south. Three sides were defended by mountain ranges: the Ered Lithui ("Ash Mountains") on the north, and the Ephel Dúath ("Mountains of Shadow") on the west and south. The lengths of these ranges are estimated to be 498, 283 and 501 miles (801, 455 and 806 kilometres) respectively, which gives Mordor an area of roughly 140,000 square miles (360,000 square kilometres).

To the west lay the narrow land of Ithilien, a province of Gondor; to the northwest, the Dead Marshes and Dagorlad, the Battle Plain; to the north, Wilderland; to the northeast and east, Rhûn; to the southeast, Khand; and to the south, Harad. Not far from the Dead Marshes is another dismal swamp, the Nindalf or Wetwang, beside the Emyn Muil hills.

In the northwest, the pass of Cirith Gorgor led into the enclosed plain of Udûn. Sauron built the Black Gate of Mordor (the Morannon) across the pass. This added to the earlier fortifications, the Towers of the Teeth – Carchost to the east, Narchost to the west, guard towers which had been built by Gondor to keep a watch on this entrance. The passage through the inner side of Udûn into the interior of Mordor was guarded by another gate, the Isenmouthe. Outside the Morannon lay the Dagorlad or Battle Plain, and the Dead Marshes.

The Ephel Dúath ("Fence of Shadow") defended Mordor on the west and south. The main pass was guarded by Minas Morgul, a city built by Gondor as Minas Ithil. The fortress Durthang lay in the northern Ephel Dúath above Udûn. A higher, more difficult pass, Cirith Ungol, lay just to the north of the Morgul pass. Its top was guarded by a tower, built by Gondor. The route traversed Torech Ungol, the lair of the giant spider Shelob.

Inside the Ephel Dúath ran a lower parallel ridge, the Morgai, separated by a narrow valley, a "dying land not yet dead" with "low scrubby trees", "coarse grey grass-tussocks", "withered mosses", "great writhing, tangled brambles", and thickets of briars with long, stabbing thorns.

The interior of Mordor was composed of three large regions. The core of Sauron's realm was in the northwest: the arid plateau of Gorgoroth, with the active volcano Mount Doom located in the middle. Sauron's main fortress Barad-dûr was on the north side of Gorgoroth, at the end of a spur of the Ash Mountains. Gorgoroth was volcanic and inhospitable to life, but home to Mordor's mines, forges, and garrisons. Núrn, the southern part of Mordor, was less arid and more fertile; Sauron's slaves farmed this region to support his armies, and streams fed the salt Sea of Núrnen. To the east of Gorgoroth lay the dry plain of Lithlad.