The SilmarillionThe Lord of the RingsUnfinished Tales

In J. R. R. Tolkien's fictional universe of Middle-earth, Mordor (from Sindarin Black Land and Quenya Land of Shadow) is the dwelling place of Sauron, in the southeast of northwestern Middle-earth to the East of Anduin, the great river. Orodruin, a volcano in Mordor, was the destination of the Fellowship of the Ring (and later Frodo Baggins and Sam Gamgee) in the quest to destroy the One Ring. Mordor was unique because of the three enormous mountain ridges surrounding it, from the North, from the West and from the South. The mountains both protected the land from an unexpected invasion by any of the people living in those directions and kept those living in Mordor from escaping. Tolkien is reported to have identified Mordor with the volcano of Stromboli off Sicily.

On four sides its walls were mountain ranges, arranged in a rough rectangle: Ered Lithui, translated as 'Ash Mountains' in the north, Ephel Dúath, translated as 'Fence of Shadow' in the west, and an unnamed (or possibly still called Ephel Dúath) range in the south. In the northwest corner of Mordor, the deep valley of Udûn formed the castle's gate and guard house. That was the only entrance for large armies, and that is where Sauron built the Black Gate of Mordor, and later where Gondor built the Towers of the Teeth. Behind the Black Gate, these towers watched over Mordor during the time of peace between the Last Alliance and Sauron's return. In front of the Morannon lay the Dagorlad or the Battle Plain.

Within this mountainous castle, Sauron's main fortress Barad-dûr formed its tower, at the foothills of Ered Lithui. To southwest of Barad-dûr lay the arid plateau of Gorgoroth, forming the castle's keep, and Mount Doom its forge. To the east lay the plain of Lithlad.

A narrow pass led through Ephel Dúath and the fortress of Minas Morgul (earlier Minas Ithil) was guarding that; an even more difficult pass was guarded by the giant spider Shelob in her lair of Torech Ungol and the fortress of Cirith Ungol. Another known fortress was Durthang in the northern Ephel Dúath.

The southern part of Mordor, Núrn, was slightly more fertile, and moist enough to carry the inland sea of Núrnen. Núrn was made somewhat fertile because the ash blown from Mount Doom left its soil nutrient rich, thus allowing dry-land farming. Unfortunately, the inland sea of Núrn was salty, not freshwater. Farming in this region supported the armies of Sauron.

To the west of Mordor was the narrow land of Ithilien with the city of Osgiliath and the great river Anduin, to the northeast Rhûn, and to the southeast, Khand. To the northwest lay the Dead Marshes.

In the chapters in The Lord of the Rings describing Frodo and Sam's journey in Mordor, the valleys in an area called the Morgai, on the land's "outer marges [...] under the westward mountains", are described as a "dying land not yet dead". The vegetation clinging to life in this area of Mordor included "low scrubby trees", "coarse grey grass-tussocks", "withered mosses", "great writhing, tangled brambles", and thickets of briars. This vegetation grew near water trickling down from higher up the valleys. Sam and Frodo sheltered under a curtain of these brambles, which had long stabbing thorns and hooked barbs. The briars also had thorns, and when Sam and Frodo fall into some briars, Sam says that the thorns feel "a foot long". The fauna described in this area included maggots, midges and flies marked with "a red eye-shaped blotch".

In The Atlas of Middle-earth, Karen Wynn Fonstad assumed that the lands of Mordor, Khand, and Rhûn lay where the inland Sea of Helcar had been, and that the Sea of Rhûn and Sea of Núrnen were its remnants. This assumption stemmed from a First Age world map drawn by Tolkien in the Ambarkanta, where the Inland Sea of Helcar was shown to occupy a large area of Middle-earth between the Ered Luin and Orocarni, with the western end being close to the head of the Great Gulf (later the Mouths of Anduin). The atlas was however published before The Peoples of Middle-earth, where it turned out that the Sea of Rhûn and Mordor existed already in the First Age, thus showing a late revision made by Tolkien towards the ancient geography of Middle-earth. However, the final state of geography devised by Tolkien is ultimately unknown, as virtually no discussion on the relationship between the Seas of Rhun and Helcar was made in his later writings.

Mordor was a relic of the devastating works of Morgoth, apparently formed by massive volcanic eruptions. It was given the name Mordor already before Sauron settled there, because of its volcano and its eruptions. However, only Shelob had settled there before Sauron did.