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 Fellowship 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 Virgil himself 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.