Isildur (Quenya: [iˈsildur]) is a fictional character in J. R. R. Tolkien's Middle-earth, the elder son of Elendil, descended from Elros, the founder of the island Kingdom of Númenor. He fled with his father when the island was drowned, becoming in his turn King of Arnor and Gondor. He cut the Ring from Sauron's hand, but instead of destroying it, was influenced by its power and claimed it for his own. He was killed by orcs, and the Ring betrayed him by slipping off his finger just before his death. The Ring then became lost in the River Anduin. This set the stage for the Ring to pass to Gollum and then to Bilbo, as told in The Hobbit; that in turn provided the central theme, the quest to destroy the Ring, for The Lord of the Rings.

Tolkien began a time-travel story, The Lost Road, in which a father and a son were to reappear time and again in human families throughout history. One of the appearances was to be in Númenor just before its fall, with the father as Elendil and the son as Herendil, later called Isildur. The story was abandoned, but Tolkien reused the characters and events.

Isildur features briefly in voiced-over flashback sequences of Peter Jackson's The Lord of the Rings film trilogy. The video game Middle-earth: Shadow of War departs from Tolkien's narrative by having Sauron make Isildur into a Nazgûl or ringwraith. In the television series The Lord of the Rings: The Rings of Power, the young Isildur is played by the English actor Maxim Baldry.

In Tolkien's legendarium, the island of Númenor, in the great sea to the West of Middle-earth, was created at the start of the Second Age as a reward to the men who had fought against the fallen Vala Morgoth, the primary antagonist of the First Age. Isildur's father was Elendil, descended from Elros, founder of the Kingdom of Númenor. Since Elros was half-elven, from the marriage of Beren to the elf Lúthien, he and his descendants enjoyed much longer life than other men.

In Isildur's youth, Ar-Pharazôn, King of Númenor, was corrupted by the fallen Maia Sauron, who urged that Nimloth the White Tree be cut down. Isildur went to the court of the king in disguise and stole a fruit of the tree. He was severely wounded during his escape, but his sacrifice was not in vain: Nimloth was cut down and burned shortly afterwards, but the line of the White Tree continued by way of the stolen fruit. When Númenor was destroyed by the creator, Ilúvatar, Elendil's family escaped in nine ships.

The refugees from Númenor fled east to the continent of Middle-earth. Isildur's father Elendil landed in the north and founded the realm of Arnor, while Isildur and his brother Anárion landed in the south, where they established the realm of Gondor and the cities of Osgiliath, Minas Ithil, and Minas Anor. Isildur lived in Minas Ithil on the east side of the River Anduin and Anárion in Minas Anor on the west side; they ruled Gondor jointly from Osgiliath.

The Dark Lord Sauron captured Minas Ithil and destroyed the White Tree Isildur had planted there. Isildur and his family escaped down the Anduin by boat, bearing with them a seedling of the tree. They sailed to Lindon, seeking the Elven King Gil-galad and Elendil in Arnor. Anárion bought time for Gondor by defending Osgiliath and driving Sauron back to the mountains, while Elendil and Gil-galad marshalled their forces.

Isildur returned with Elendil and Gil-galad in the Last Alliance of Elves and Men. After the Alliance defeated Sauron's host at the Battle of Dagorlad, they advanced into Mordor and laid siege to Barad-dûr. When Minas Ithil was recaptured, Isildur sent his younger sons Aratan and Ciryon to man that fortress, preventing Sauron and his forces from escaping that way. Isildur was accompanied throughout the war by his eldest son Elendur. The campaign in Mordor was long and bitter, and Anárion was killed by a stone from the Dark Tower.