Sauron (/ˈsaʊərɒn/) is the title character and the main antagonist in J. R. R. Tolkien's The Lord of the Rings, where he rules the land of Mordor. He has the ambition of ruling the whole of Middle-earth using the power of the One Ring, which he has lost and seeks to recapture. In the same work, he is identified as the "Necromancer" of Tolkien's earlier novel The Hobbit. The Silmarillion describes him as the chief lieutenant of the first Dark Lord, Morgoth. Tolkien noted that the Ainur, the "angelic" powers of his constructed myth, "were capable of many degrees of error and failing", but by far the worst was "the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron". Sauron appears most often as "the Eye", as if disembodied.

Tolkien, while denying that absolute evil could exist, stated that Sauron came as near to a wholly evil will as was possible. Commentators have compared Sauron to the title character of Bram Stoker's 1897 novel Dracula, and to Balor of the Evil Eye in Irish mythology. Sauron is briefly seen in a humanoid form in Peter Jackson's film trilogy, which otherwise shows him as a disembodied, flaming Eye.

The Ainulindalë tells how the supreme being Eru began the creation with good, immortal spirits, the Ainur. The lesser Maiar included Sauron, under the greater Valar. The Vala Melkor rebelled against Eru, starting evils that Sauron continued. Sauron perceived Eru directly; he was "far higher" than the Maiar who later came to Middle-earth as Wizards.

Sauron served Aulë, the smith of the Valar, acquiring knowledge; he was called Mairon (Quenya: "The Admirable") until he betrayed the Valar by joining Melkor. In Beleriand, he was called Gorthu (Sindarin: "Mist of Fear") and Gorthaur (Sindarin: "The Cruel"). Sauron, hating disorder, was drawn to Melkor's power. He became a spy for Melkor on the isle of Almaren, the Valar's home, which Melkor soon destroyed; the Valar moved to Valinor, not perceiving Sauron's treachery. Sauron followed Melkor to Middle-earth, joining the Valar's enemy.

Sauron helped Melkor in every kind of deceit. By the time Elves awoke, Sauron was Melkor's lieutenant with command over the stronghold of Angband. The Valar made war on and captured Melkor; Sauron escaped. He repaired Angband, and bred an army of Orcs. Melkor, now called Morgoth, murdered Finwë, King of the Noldor, and escaped to Middle-earth with the Silmarils, pursued by the Noldor. Sauron directed the war against the Elves, conquering their fortress of Minas Tirith (not the later city in Gondor) on the isle of Tol Sirion. The elf Lúthien came there to save her lover, the imprisoned Beren, with Huan the Wolfhound. Sauron, as a werewolf, battled Huan, who took him by the throat; Sauron was defeated and fled, taking the form of a huge vampire bat. Lúthien destroyed the tower and rescued Beren. Later, the half-elf Eärendil sailed to Valinor to ask the Valar to fight Morgoth. They did so in the War of Wrath, and Morgoth was defeated and cast into the Outer Void. Again, however, Sauron escaped.

In the Second Age, Sauron reappeared, intent on taking over Middle-earth. To seduce the Elves into his service, Sauron assumed a fair appearance as Annatar, "Lord of Gifts", and befriended Celebrimbor's Elven-smiths. He taught them arts and magic, helping them to forge the Rings of Power. Sauron then secretly forged the One Ring to rule all the others. The Elves detected him when he put on the Ring, and removed their Rings. Enraged, Sauron made war, killed Celebrimbor, and seized the Seven and the Nine Rings of Power. The Three Rings were hidden by the Elves Gil-galad, Círdan, and Galadriel. Sauron attacked them. The Elves were saved by an army from Númenor, defeating Sauron. Sauron fortified Mordor and completed the Dark Tower, Barad-dûr. He distributed the Seven and the Nine Rings to lords of Dwarves and Men. Dwarves did not submit, but he enslaved nine Men as the feared Nazgûl. Orcs, Trolls, Easterlings and men of Harad became his servants.

Late in the Second Age, the men of Númenor sought to colonise Middle-earth. Led by Ar-Pharazôn, a massive army sailed to Middle-earth to battle Sauron. Dismayed, Sauron surrendered, hoping to corrupt Númenor from within. Using the One Ring, Sauron soon exerted a malign influence over the Númenóreans, undermining Númenor's religion and deceiving its people into worshiping Melkor with human sacrifice. Sauron later deceived Ar-Pharazôn into attacking Aman by sea to steal immortality from the Valar. The Valar appealed to Eru, who destroyed Númenor. Sauron's body was destroyed, and he lost the ability to appear beautiful.

Led by Elendil, nine ships escaped to Middle-earth from the Downfall of Númenór. There, the Númenóreans founded the kingdoms of Gondor and Arnor. Sauron returned to Mordor, took on a new physical form, and made war on these Exiles. He captured the fortress of Minas Ithil, and Elendil's son Isildur escaped down the Anduin River. Elendil's other son, Anárion, defended Osgiliath, the capital city of Gondor, and drove Sauron's forces back to the mountains. Elendil, Isildur and Anárion formed the Last Alliance with the Elves and defeated Sauron at Dagorlad. They invaded Mordor and besieged Barad-dûr for seven years. Finally, Sauron came out to fight face-to-face, killing Elendil and Gil-galad; Elendil's sword Narsil broke beneath him. Isildur took up the hilt-shard and cut the One Ring from Sauron's hand, vanquishing Sauron. Isildur refused to destroy the Ring by casting it into Mount Doom, but kept it for his own.