The Nazgûl , introduced as Black Riders and also called Ringwraiths, Dark Riders, the Nine Riders, or simply the Nine, are fictional characters in J. R. R. Tolkien's Middle-earth. They were nine Men who had succumbed to Sauron's power through wearing Rings of Power, which gave them immortality but reduced them to invisible wraiths, servants bound to the power of the One Ring and completely under Sauron's control.

The Lord of the Rings calls them Sauron's most terrible servants. Their leader, known as the Lord of the Nazgûl or the Witch-king of Angmar, had once been the King of Angmar in the north of Eriador. At the end of the Third Age, their main stronghold was the city of Minas Morgul at the entrance to Sauron's realm, Mordor. They dress entirely in black. In their early forays, they ride on black horses; later they ride flying monsters, which Tolkien described as pterodactylic. Their main weapon is terror, though in their pursuit of the Ring-bearer Frodo, their leader uses a Morgul-knife which would reduce its victim to a wraith, and they carry ordinary swords. In his final battle, the Lord of the Nazgûl attacks Éowyn with a mace. The hobbit Merry stabs him with an ancient enchanted Númenórean blade, allowing Éowyn to kill him with her sword.

Commentators have written that the Nazgûl serve on the ordinary level of story as dangerous opponents; at the romantic level as the enemies of the heroic protagonists; and at the mythic level, where the Lord of the Nazgûl works magic such as bursting the gates of Minas Tirith with magical spells, and calls himself Death. The prophecy that the Lord of the Nazgûl would not die by the hand of man echoes that made of Macbeth in Shakespeare's play.

The Nazgûl appear in numerous adaptations of Tolkien's writings, including animated and live-action films and computer games. Species of several different kinds of animal, including an ant, a crustacean, and a parasitoid wasp, have been named after them.