In J. R. R. Tolkien's fantasy universe of Middle-earth, Isildur was a Dúnadan of Númenor, elder son of Elendil.

Isildur was born in the year 3220 of the Second Age in Númenor as first son of Elendil, son of Amandil the last Lord of Andúnie. He had a younger brother Anárion. Isildur had four sons: Elendur, Aratan, Ciryon, and Valandil.

In his youth, Isildur stole a fruit of Nimloth before it was cut down, preserving the line of the White Tree, and later planted a seedling in the city of Minas Anor.

Isildur together with his father and brother were saved from the Downfall of Númenor when they fled for Middle-earth. Isildur and Anárion landed in the south and established the realm of Gondor, and their father landed in the north, founding the realm of Arnor.

Isildur lived on the east bank of the Anduin, and established the city of Minas Ithil, as well as the province of Ithilien. However, in 3428 Sauron attacked and took Minas Ithil, and Isildur fled north to his father, leaving Anárion to rule over Gondor.

He returned with his father and the Elven High King Gil-galad in the Last Alliance of Elves and Men in 3434. His father and brother were killed as was Gil-galad, but Isildur took the shards of his father's sword Narsil and cut the One Ring from Sauron's finger, destroying Sauron's physical form and winning the war.

Despite the advice of Elrond and Círdan, lieutenant of Gil-galad, he did not destroy the Ring, instead claiming it as an heirloom for his House.

After the fall of Sauron, Isildur left Meneldil, son of his brother Anárion in charge of Gondor, and returned north to Arnor with his three sons. His fourth son Valandil had stayed behind in Rivendell. At the Gladden Fields, Isildur was ambushed by roaming Orcs, and due to the Ring's treachery he was killed, losing the Ring in the Gladden river.

He was the last king to rule both Gondor and Arnor until King Elessar reunited the Kingdom in the Fourth Age.

In the riddling rhyme "Seek for the Sword that was Broken" the Ring is referred to cryptically as Isildur's Bane.

In the Lord of the Rings movie trilogy by Peter Jackson, Isildur briefly appears in the first scenes of the first movie, and in an extended flashback scene in the second movie. In the movie, Isildur apparently was the last king of Gondor (Anárion and heirs do not appear at all), and Arnor does not feature at all: furthermore the time between the Fall of Sauron and the War of the Ring is much shorter. The story of Isildur's succumbing to the temptation of the Ring bothers Aragorn who fears that he could have the same weakness.