The Nazgûl (from Black Speech nazg 'ring', and gûl 'wraith, spirit') – 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 Witch-king of Angmar, the Lord of the Nazgûl, or the Black Captain, was Sauron's chief agent for most of the Third Age. 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 Baggins, 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 Brandybuck 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 of the Company of the Ring; at the romantic level as the enemies of the heroic protagonists; and finally at the mythic level. Tolkien knew the Lacnunga, the Old English book of spells; it may have suggested multiple features of the Nazgûl, the Witch-King, and the Morgul-knife.

The Nazgûl appear in numerous adaptations of Tolkien's writings, including animated and live-action films and computer games.

Those who used the Nine Rings became mighty in their day, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their downfall. They had, as it seemed, unending life, yet life became unendurable to them. They could walk, if they would, unseen by all eyes in this world beneath the sun, and they could see things in worlds invisible to mortal men; but too often they beheld only the phantoms and delusions of Sauron. And one by one, sooner or later, according to their native strength and to the good or evil of their wills in the beginning, they fell under the thraldom of the ring that they bore and of the domination of the One which was Sauron's. And they became forever invisible save to him that wore the Ruling Ring, and they entered into the realm of shadows. The Nazgûl were they, the Ringwraiths, the Úlairi, the Enemy's most terrible servants; darkness went with them, and they cried with the voices of death.

The Nazgûl or Ringwraiths (Quenya plural: Úlairi) first appeared in the Second Age. The Dark Lord Sauron gave nine Rings of Power to powerful mortal men, including three lords of the once-powerful island realm of Númenor, along with kings of countries in Middle-earth. The rings enslaved their bearers to the power of Sauron's One Ring, into which he had put much of his own power. The corrupting effect of the Rings greatly extended the bearers' lives.

The Nazgûl had a sharp sense of smell. Their sight worked differently, too: "They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys; and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared." Their chief weapon was terror; it was so powerful that Sauron faced one disadvantage when using them: they could not easily travel in secret. The terror they spread was greater when they were unclad and invisible; and when they were gathered together.

Only two of the Nazgûl are named or identified individually in Tolkien's works. Their chief, also known as the Lord of the Nazgûl and the Black Captain, appears as the Witch-king of Angmar during the Third Age, instrumental in the destruction of the North-kingdom of Arnor. In Unfinished Tales, his second-in-command is named as Khamûl, the "Black Easterling" or the "Shadow of the East". Three of the Nazgûl were great Númenórean lords; in his notes for translators, Tolkien speculated that the Witch-king of Angmar, ruler of a northern kingdom with its capital at Carn Dûm, was of Númenórean origin.