Gandalf is a protagonist in J. R. R. Tolkien's novels The Hobbit and The Lord of the Rings. He is a wizard, one of the Istari order, and the leader of the Company of the Ring. Tolkien took the name "Gandalf" from the Old Norse "Catalogue of Dwarves" (Dvergatal) in the Völuspá.

As a wizard and the bearer of one of the Three Rings, Gandalf has great power, but works mostly by encouraging and persuading. He sets out as Gandalf the Grey, possessing great knowledge and travelling continually. Gandalf is focused on the mission to counter the Dark Lord Sauron by destroying the One Ring. He is associated with fire; his ring of power is Narya, the Ring of Fire. As such, he delights in fireworks to entertain the hobbits of the Shire, while in great need he uses fire as a weapon. As one of the Maiar, he is an immortal spirit from Valinor, but his physical body can be killed.

In The Hobbit, Gandalf assists the 13 dwarves and the hobbit Bilbo Baggins with their quest to retake the Lonely Mountain from Smaug the dragon, but leaves them to urge the White Council to expel Sauron from his fortress of Dol Guldur. In the course of the quest, Bilbo finds a magical ring. The expulsion succeeds, but in The Lord of the Rings, Gandalf reveals that Sauron's retreat was only a feint, as he soon reappeared in Mordor. Gandalf further explains that, after years of investigation, he is sure that Bilbo's ring is the One Ring that Sauron needs to dominate the whole of Middle-earth. The Council of Elrond creates the Fellowship of the Ring, with Gandalf as its leader, to defeat Sauron by destroying the Ring. He takes them south through the Misty Mountains, but is killed fighting a Balrog, an evil spirit-being, in the underground realm of Moria. After he dies, he is sent back to Middle-earth to complete his mission as Gandalf the White. He reappears to three of the Fellowship and helps to counter the enemy in Rohan, then in Gondor, and finally at the Black Gate of Mordor, in each case largely by offering guidance. When victory is complete, he crowns Aragorn as King before leaving Middle-earth for ever to return to Valinor.

Tolkien once described Gandalf as an angel incarnate; later, both he and other scholars have likened Gandalf to the Norse god Odin in his "Wanderer" guise. Others have described Gandalf as a guide-figure who assists the protagonists, comparable to the Cumaean Sibyl who assisted Aeneas in Virgil's The Aeneid, or to the figure of Virgil in Dante's Inferno. Scholars have likened his return in white to the transfiguration of Christ; he is further described as a prophet, representing one element of Christ's threefold office of prophet, priest, and king, where the other two roles are taken by Frodo and Aragorn.

The Gandalf character has been featured in radio, television, stage, video game, music, and film adaptations, including Ralph Bakshi's 1978 animated film. His best-known portrayal is by Ian McKellen in Peter Jackson's 2001–2003 The Lord of the Rings film series, where the actor based his acclaimed performance on Tolkien himself. McKellen reprised the role in Jackson's 2012–2014 film series The Hobbit.

Tolkien derived the name Gandalf from Gandálfr, a dwarf in the Völuspá's Dvergatal, a list of dwarf-names. In Old Norse, the name means staff-elf. This is reflected in his name Tharkûn, which is "said to mean 'Staff-man'" in Khuzdul, the language Tolkien invented for his Dwarves.

Gandalf is given several names and epithets in Tolkien's writings. Faramir calls him the Grey Pilgrim, and reports Gandalf as saying, "Many are my names in many countries. Mithrandir among the Elves, Tharkûn to the Dwarves, Olórin I was in my youth in the West that is forgotten, in the South Incánus, in the North Gandalf; to the East I go not." In an early draft of The Hobbit, he is called Bladorthin, while the name Gandalf is used by the dwarf who later became Thorin Oakenshield.

Each Wizard is distinguished by the colour of his cloak. For most of his manifestation as a wizard, Gandalf's cloak is grey, hence the names Gandalf the Grey and Greyhame, from Old English hama, "cover, skin". Mithrandir is a name in Sindarin meaning "Grey Pilgrim" or "Grey Wanderer". Midway through The Lord of the Rings, Gandalf becomes the head of the order of Wizards, and is renamed Gandalf the White. This change in status (and clothing) introduces another name for the wizard: the White Rider. However, characters who speak Elvish still refer to him as Mithrandir. At times in The Lord of the Rings, other characters address Gandalf by insulting nicknames: Stormcrow, Láthspell ("Ill-news" in Old English), and "Grey Fool".

Tolkien describes Gandalf as the last of the wizards to appear in Middle-earth, one who "seemed the least, less tall than the others, and in looks more aged, grey-haired and grey-clad, and leaning on a staff". Yet the Elf Círdan who met him on arrival nevertheless considered him "the greatest spirit and the wisest" and gave him the Elven Ring of Power called Narya, the Ring of Fire, containing a "red" stone for his aid and comfort. Tolkien explicitly links Gandalf to the element fire later in the same essay:

Warm and eager was his spirit (and it was enhanced by the ring Narya), for he was the Enemy of Sauron, opposing the fire that devours and wastes with the fire that kindles, and succours in wanhope and distress; but his joy, and his swift wrath, were veiled in garments grey as ash, so that only those that knew him well glimpsed the flame that was within. Merry he could be, and kindly to the young and simple, yet quick at times to sharp speech and the rebuking of folly; but he was not proud, and sought neither power nor praise ... Mostly he journeyed tirelessly on foot, leaning on a staff, and so he was called among Men of the North Gandalf 'the Elf of the Wand'. For they deemed him (though in error) to be of Elven-kind, since he would at times work wonders among them, loving especially the beauty of fire; and yet such marvels he wrought mostly for mirth and delight, and desired not that any should hold him in awe or take his counsels out of fear. ... Yet it is said that in the ending of the task for which he came he suffered greatly, and was slain, and being sent back from death for a brief while was clothed then in white, and became a radiant flame (yet veiled still save in great need).

In Valinor, Gandalf was called Olórin. He was one of the Maiar of Valinor, specifically, one of the people of the Vala Manwë; he was said to be the wisest of the Maiar. He was closely associated with two other Valar: Irmo, in whose gardens he lived, and Nienna, the patron of mercy, who gave him tutelage. When the Valar decided to send the order of the Wizards (Istari) across the Great Sea to Middle-earth to counsel and assist all those who opposed Sauron, Olórin was proposed by Manwë. Olórin initially begged to be excused, declaring he was too weak and that he feared Sauron, but Manwë replied that that was all the more reason for him to go.

As one of the Maiar, Gandalf was not a mortal Man but an angelic being who had taken human form. As one of those spirits, Olórin was in service to the Creator (Eru Ilúvatar) and the Creator's 'Secret Fire'. Along with the other Maiar who entered into Middle-earth as the five Wizards, he took on the specific form of an old man as a sign of his humility. The role of the wizards was to advise and counsel but never to attempt to match Sauron's strength with their own. It might be, too, that the kings and lords of Middle-earth would be more receptive to the advice of a humble old man than a more glorious form giving them direct commands.

The wizards arrived in Middle-earth separately, early in the Third Age; Gandalf was the last, landing in the Havens of Mithlond. He seemed the oldest and least in stature, but Círdan the Shipwright felt that he was the greatest on their first meeting in the Havens, and gave him Narya, the Ring of Fire. Saruman, the chief Wizard, learned of the gift and resented it. Gandalf hid the ring well, and it was not widely known until he left with the other ring-bearers at the end of the Third Age that he, and not Círdan, was the holder of the third of the Elven-rings.

Gandalf's relationship with Saruman, the head of their Order, was strained. The Wizards were commanded to aid Men, Elves, and Dwarves, but only through counsel; they were forbidden to use force to dominate them, though Saruman increasingly disregarded this.

Gandalf suspected early on that an evil presence, the Necromancer of Dol Guldur, was not a Nazgûl but Sauron himself. He went to Dol Guldur to discover the truth, but the Necromancer withdrew before him, only to return with greater force, and the White Council was formed in response. Galadriel had hoped Gandalf would lead the council, but he refused, declining to be bound by any but the Valar who had sent him. Saruman was chosen instead, as the most knowledgeable about Sauron's work in the Second Age.

Gandalf returned to Dol Guldur "at great peril" and learned that the Necromancer was indeed Sauron. The following year a White Council was held, and Gandalf urged that Sauron be driven out. Saruman, however, reassured the Council that Sauron's evident effort to find the One Ring would fail, as the Ring would long since have been carried by the river Anduin to the Sea; and the matter was allowed to rest. But Saruman began actively seeking the Ring near the Gladden Fields where Isildur had been killed.

"The Quest of Erebor" in Unfinished Tales elaborates upon the story behind The Hobbit. It tells of a chance meeting between Gandalf and Thorin Oakenshield, a Dwarf-king in exile, in the Prancing Pony inn at Bree. Gandalf had for some time foreseen the coming war with Sauron, and knew that the North was especially vulnerable. If Rivendell were to be attacked, the dragon Smaug could cause great devastation. He persuaded Thorin that he could help him regain his lost territory of Erebor from Smaug, and so the quest was born.

Gandalf meets with Bilbo in the opening of The Hobbit. He arranges for a tea party, to which he invites the thirteen dwarves, and thus arranges the travelling group central to the narrative. Gandalf contributes the map and key to Erebor to assist the quest. On this quest Gandalf acquires the sword, Glamdring, from the trolls' treasure hoard. Elrond informs them that the sword was made in Gondolin, a city long ago destroyed, where Elrond's father lived as a child.

After escaping from the Misty Mountains pursued by goblins and wargs, the party is carried to safety by the Great Eagles. Gandalf then persuades Beorn to house and provision the company for the trip through Mirkwood. Gandalf leaves the company before they enter Mirkwood, saying that he had pressing business to attend to.

He turns up again before the walls of Erebor disguised as an old man, revealing himself when it seems the Men of Esgaroth and the Mirkwood Elves will fight Thorin and the dwarves over Smaug's treasure. The Battle of Five Armies ensues when hosts of goblins and wargs attack all three parties. After the battle, Gandalf accompanies Bilbo back to the Shire, revealing at Rivendell what his pressing business had been: Gandalf had once again urged the council to evict Sauron, since quite evidently Sauron did not require the One Ring to continue to attract evil to Mirkwood. Then the Council "put[s] forth its power" and drives Sauron from Dol Guldur. Sauron had anticipated this, and had feigned a withdrawal, only to reappear in Mordor.

Gandalf spent the years between The Hobbit and The Lord of the Rings travelling Middle-earth in search of information on Sauron's resurgence and Bilbo Baggins's mysterious ring, spurred particularly by Bilbo's initial misleading story of how he had obtained it as a "present" from Gollum. During this period, he befriended Aragorn and became suspicious of Saruman. He spent as much time as he could in the Shire, strengthening his friendship with Bilbo and Frodo, Bilbo's orphaned cousin and adopted heir.

Gandalf returns to the Shire for Bilbo's "eleventy-first" (111th) birthday party, bringing many fireworks for the occasion. After Bilbo, as a prank on his guests, puts on the ring and disappears, Gandalf urges his old friend to leave the ring to Frodo, as they had planned. Bilbo becomes hostile, accusing Gandalf of trying to steal the ring. Alarmed, Gandalf tells Bilbo that is foolish. Coming to his senses, Bilbo admits that the ring has been troubling him, and leaves it behind for Frodo as he departs for Rivendell.

Over the next 17 years, Gandalf travels extensively, searching for answers on the ring. He finds some answers in Isildur's scroll, in the archives of Minas Tirith. Gandalf searches long and hard for Gollum, often assisted by Aragorn, who eventually succeeds in capturing Gollum. Gandalf questions Gollum, threatening him with fire when he proves unwilling to speak. Gandalf learns that Sauron had imprisoned Gollum in his fortress of Barad-dûr, and tortured him to reveal what he knew of the Ring.

Returning to the Shire, Gandalf confirms his suspicion by throwing the Ring into Frodo's hearth-fire and reading the writing that appears on its surface. He tells Frodo the history of the ring, and urges him to take it to Rivendell, warning of grave danger if he stays in the Shire. Gandalf says he will attempt to return for Frodo's 50th birthday party, to accompany him on the road; and that meanwhile Frodo should arrange to leave quietly, as the servants of Sauron will be searching for him.

Outside the Shire, Gandalf encounters the wizard Radagast the Brown, who brings the news that the Nazgûl have ridden out of Mordor—and a request from Saruman that Gandalf come to Isengard. Gandalf asks him to send out animals to observe the Nazgûl, and to report to him at Isengard. Gandalf leaves a letter to Frodo (urging his immediate departure) with Barliman Butterbur at the Prancing Pony, and heads towards Isengard. There, Saruman horrifies Gandalf by asking him to help him to obtain and use the Ring. Gandalf refuses, and Saruman imprisons him at the top of his tower. Gandalf is rescued by Gwaihir the Eagle, who comes to him as requested via Radagast.

In Rohan, Gandalf appeals to King Théoden for a horse. Théoden, under the evil influence of Gríma Wormtongue, Saruman's spy and servant, tells Gandalf to take any horse he pleases, but to leave quickly. It is then that Gandalf meets the great horse Shadowfax, who will be his mount and companion. Gandalf rides hard for the Shire, but does not reach it until after Frodo has set out. Knowing that Frodo and his companions will be heading for Rivendell, Gandalf makes his own way there. He learns at Bree that the Hobbits have fallen in with Aragorn. He faces the Nazgûl at Weathertop, but escapes after an all-night battle, drawing four of them northward. Frodo, Aragorn and company face the remaining five on Weathertop a few nights later. Gandalf reaches Rivendell just before Frodo's arrival.

In Rivendell, Gandalf helps Elrond drive off the Nazgûl pursuing Frodo, and plays a leading role in the Council of Elrond as the only person who knows the full history of the ring. He reveals that Saruman has betrayed them and is in league with Sauron. When it is decided that the Ring has to be destroyed, Gandalf volunteers to accompany Frodo—now the Ring-bearer—in his quest. He persuades Elrond to let Frodo's cousins Merry and Pippin join the Fellowship.

Taking charge of the Fellowship (comprising nine representatives of the free peoples of Middle-earth, "set against the Nine Riders"), Gandalf and Aragorn lead the Hobbits and their companions south. After an unsuccessful attempt to cross Mount Caradhras in winter, they cross under the mountains through the Mines of Moria under the Misty Mountains, though only Gimli the Dwarf is enthusiastic about that route. In Moria, they discover that the dwarf colony established there by Balin has been annihilated by orcs. The Fellowship fights with the orcs and trolls of Moria and escapes them.

At the Bridge of Khazad-dûm, they encounter "Durin's Bane," a fearsome Balrog from ancient times. Gandalf faces the Balrog to enable the others to escape. After a brief exchange of blows, Gandalf breaks the bridge beneath the Balrog with his staff. As the Balrog falls, it wraps its whip around Gandalf's legs, dragging him over the edge. Gandalf falls into the abyss, crying "Fly, you fools!".

Gandalf and the Balrog fall into a deep lake in Moria's underworld. Gandalf pursues the Balrog through the tunnels for eight days until they climb to the peak of Zirakzigil. Here they fight for two days and nights. The Balrog is defeated and cast down onto the mountainside. Gandalf also dies, and his body lies on the peak while his spirit travels "out of thought and time".

Gandalf is "sent back" as Gandalf the White, and returns to life on the mountain top. Gwaihir carries him to Lothlórien, where he is healed of his injuries and re-clothed in white robes by Galadriel. He travels to Fangorn Forest, where he encounters Aragorn, Gimli, and Legolas (who are tracking Merry and Pippin). They mistake him for Saruman, but he stops their attacks and reveals himself.

They travel to Rohan, where Gandalf finds that Théoden has been further weakened by Wormtongue's influence. He breaks Wormtongue's hold over Théoden, and convinces the king to join in the fight against Sauron. Gandalf sets off to gather warriors of the Westfold to assist Théoden in the coming battle with Saruman. Gandalf arrives just in time to defeat Saruman's army in the battle of Helm's Deep. Gandalf and the King ride to Isengard, which has just been destroyed by Treebeard and his Ents, who are accompanied by Merry and Pippin. Gandalf breaks Saruman's staff and expels him from the White Council and the Order of Wizards; Gandalf takes Saruman's place as head of both. Wormtongue tries to kill Gandalf or Saruman with the palantír of Orthanc, but misses both. Pippin retrieves the palantír, but Gandalf quickly takes it. After the group leaves Isengard, Pippin takes the palantír from a sleeping Gandalf, looks into it, and comes face to face with Sauron himself. Gandalf gives the palantír to Aragorn and takes the chastened Pippin with him to Minas Tirith to keep the young Hobbit out of further trouble.

Gandalf arrives in time to help to arrange the defences of Minas Tirith. His presence is resented by Denethor, the Steward of Gondor; but when his son Faramir is gravely wounded in battle, Denethor sinks into despair and madness. Together with Prince Imrahil, Gandalf leads the defenders during the siege of the city. When the forces of Mordor break the main gate, Gandalf, alone on Shadowfax, confronts the Lord of the Nazgûl. At that moment the Rohirrim arrive, causing the Nazgûl to withdraw. Gandalf is about to pursue, but is stopped by Pippin, who requests his intervention to save Faramir – Denethor in desperation was seeking to burn himself and his son on a funeral pyre. Gandalf saves Faramir (but not Denethor, who immolates himself), and plays no further part in the unfolding Battle of the Pelennor Fields.

After the battle, Gandalf counsels an attack against Sauron's forces at the Black Gate, in an effort to distract the Dark Lord's attention from Frodo and Sam; they are at that moment scaling Mount Doom to destroy the Ring. Gandalf, Aragorn and the other leaders of the West lead an army to the Black Gate, meeting the nameless lieutenant of Mordor, who shows them Frodo's mithril shirt and other items from the Hobbits' equipment. Gandalf rejects Mordor's terms of surrender, starting the Battle of the Morannon. The forces of the West face the full might of Sauron's armies, until the Ring is destroyed in Mount Doom. Gandalf leads the Eagles to rescue Frodo and Sam from the erupting mountain.

After the war, Gandalf crowns Aragorn as King Elessar, and helps him find a sapling of the White Tree of Gondor. He accompanies the Hobbits back to the borders of the Shire, before leaving to visit Tom Bombadil.

Two years later, Gandalf departs Middle-earth forever. He boards the Ringbearers' ship in the Grey Havens and sets sail to return across the sea to the Undying Lands; with him are his horse Shadowfax and his friends Frodo, Bilbo, Galadriel, and Elrond.