Thranduil is a fictional character in J. R. R. Tolkien's Middle-earth legendarium. He first appears as a supporting character in The Hobbit, where he is simply known as the Elvenking, the ruler of the Elves who lived in the woodland realm of Mirkwood. The character is properly named in Tolkien's The Lord of the Rings, and appears briefly in The Silmarillion and Unfinished Tales.

The character has appeared in adaptations of The Hobbit in other media. The 2010s film adaptations of The Hobbit expands the character's role within the narrative, using information from Tolkien's later works about the character and original material by the filmmakers. Thranduil is portrayed by the American actor Lee Pace; he has been well received by fans and critics.

Thranduil is one of the Sindar or Grey Elves who speak Sindarin as opposed to Quenya, the language spoken by Noldorin Elves like Galadriel. The name "Thranduil" means "vigorous spring" in Sindarin. Following the end of the First Age and the destruction of much of Beleriand during the War of Wrath, many Sindar migrate into the east of Middle-earth. Crossing the Misty Mountains, they found populations of Silvan Elves living in the woodlands that bordered the River Anduin. The Sindar were welcomed by these people, and some were made leaders and rulers over them. In Appendix B of The Lord of the Rings Tolkien states that Thranduil was one of the Sindar who migrated eastward early in the Second Age and established kingdoms among the Silvan Elves. Later writings in Unfinished Tales make Thranduil's father Oropher the founder of the realm, which originally included the southern portions of the forest of Greenwood the Great, later known as Mirkwood.

Tolkien described Thranduil in The Hobbit as having "golden hair" with a crown made of red leaves and berries in the autumn, and wearing a similar crown of flowers in the spring.

Thranduil marched with his father and a large army of their people to join the Last Alliance of Elves and Men in their war against Sauron at the end of the Second Age. The Elves suffered serious losses, including Oropher, who was killed in the Battle of Dagorlad before the Black Gate of Mordor. Following the war, Thranduil, now king of his people, led the remnants of his army, only a third of what had set out, back to their woodland home in Mirkwood. Hearing word of the Disaster of the Gladden Fields shortly after their return, Thranduil set out to aid the Dúnedain; he arrived too late to save them, but was able to finish the destruction of the Orc horde and prevent the mutilation of the dead.

During the Third Age, Thranduil led his people to the north-east corner of the forest and there created an underground fortress and series of great halls. He was inspired in this enterprise by Thingol's halls of Menegroth in Doriath during the First Age, and like Thingol, he used the skill of the Dwarves to aid in making his stronghold.

Being far in the north, and on the eastern edge of an increasingly perilous Mirkwood, Thranduil's realm was somewhat isolated, but he traded with the Dwarves and Men who lived nearby in Erebor, Dale, and Esgaroth (Laketown). An attack by the dragon Smaug destroyed Erebor and Dale, and reduced Esgaroth to a shell of its former self, though there remained a healthy wine trade between the lake and the wood. This situation changed with the arrival of Bilbo Baggins and a company of Dwarves, on their quest to reclaim Erebor. The Dwarves were captured by Thranduil's guards and, suspicious of their intentions, he had them locked in his dungeons from which they later escaped inside barrels.

In a great hall with pillars hewn out of the living stone sat the Elvenking on a chair of carven wood. On his head was a crown of berries and red leaves, for the autumn was come again. In the spring he wore a crown of woodland flowers. In his hand he held a carven staff of oak. – The Hobbit: "Barrels out of Bond"