In the fictional fantasy-world of J. R. R. Tolkien, the War of the Ring was fought between Sauron and the free peoples of Middle-earth for control of the One Ring and dominion over the continent. The War of the Ring took place at the end of the Third Age. Together with the Quest of Mount Doom, it is one of the overarching events of The Lord of the Rings. Gandalf led the free peoples of Middle Earth to victory over the Dark Lord.

The war was initiated by Sauron, who had gained strength since the end of the Second Age and sought the One Ring he had forged and into which he had invested much of his power, and that he had lost in the climactic battle at the end of the prior age. During the War of the Ring, many thousands of Men of Rohan and Gondor were killed. The overall number of combatants in the war is estimated to be over a million—tens of thousands from the side of the Free Peoples and hundreds of thousands from the armies and navies of those loyal to the Enemy. The war also heralded the decline of the Elves' power in Middle-earth, the rise of Men in the West, the restoration of the King of Gondor and Arnor and the start of the Fourth Age.

Battles were fought in Gondor, Rohan, Lothlórien, Mirkwood, at the Lonely Mountain and at Dale. These were primarily waged against Sauron's forces, but Saruman, a third contender, also had armies, who fought battles at the Fords of Isen and Helm's Deep.

The war ended after the Battle of Bywater and, shortly afterwards, the deaths of Saruman and Gríma Wormtongue. Towards the end of the War of the Ring, Elessar was crowned King of Gondor, and forgave the Men who had fought under Sauron, heralding a great renewal of cooperation and communication between Men, Elves, and Dwarves.