The Silmarils (Quenya in-universe pl. Silmarilli [sil.maˈril.li], lit. 'radiance of pure light') are three fictional brilliant jewels in J. R. R. Tolkien's legendarium, made by the Elf Fëanor, capturing the pure light of the Two Trees of Valinor. The Silmarils play a central role in Tolkien's book The Silmarillion, which tells of the creation of Eä (the universe) and the beginning of Elves, Dwarves and Men.

Tolkien, a philologist, derived the idea of Silmarils, jewels that actually contained light, from the Old English word Siġelwara; he concluded that Siġel meant both 'sun' and 'jewel'. Scholars have remarked on their similarities to the Sampo in the Kalevala and to the Holy Grail of Arthurian legend. They have described the Silmarils as embodying Elvish pride in their own creation, or a Biblical desire for knowledge of good and evil, as in the Genesis story of the Garden of Eden. Verlyn Flieger analyzes The Silmarillion as a story of splintering of the created light, which in her view Tolkien equates with God. The light is embodied first in two great lamps atop tall pillars to light Middle-earth. When these are destroyed, the light is held in the Two Trees of Valinor, and Fëanor crafts the Silmarils using their light. When the trees are killed, the last available splinters of the created light in Middle-earth are the Silmarils. When the Silmarils are scattered, to Earth, Sea, and Sky, the skyborne one becomes Eärendil's Star. The Elf-lady Galadriel collects light from the star and captures a little of it in the Phial of Galadriel, which enables the Hobbit protagonists of The Lord of the Rings to fulfil their quest.

J. R. R. Tolkien describes the history of the Silmarils in The Silmarillion, published after but in fiction long preceding the events of The Lord of the Rings. The Silmarils—"the most renowned of all the works of the Elves"—are created by Fëanor, a prince of the most skilful clan of Elves, the Noldor, from the light of the Two Trees of Valinor. The Silmarils are made of the crystalline substance silima. The Silmarils are hallowed by the Vala Varda, who kindled the first stars, so that they would burn the hands of any evil creature or mortal who touched them without just cause.

Together with the evil spider Ungoliant, the rebellious Vala Melkor destroys the Two Trees of Valinor. Later, at the healing effort of the Valar, one of the trees bears a silver flower, and the other bears a golden fruit, just before they die. These are sent to the sky, and became the Sun and the Moon, to illuminate Middle-earth against Melkor. But neither sphere radiates the original light of the trees, free of Ungoliant's poison. The Silmarils then contain all the remaining unmarred light of the Two Trees. Therefore, the Valar entreat Fëanor to give them up so they can restore the Trees, but he refuses. Then news comes that Melkor has killed Fëanor's father Finwë, the High King of the Noldor, and stolen the Silmarils. After this deed, Melkor flees from Valinor to his fortress Angband in the north of Middle-earth. Thereafter he wears the Silmarils in his Iron Crown.

Fëanor is furious at Melkor, whom he names Morgoth, "Dark Enemy of the World", and at the Valar's desire to take the gems for their own purposes. Together with his seven sons he swears the Oath of Fëanor, which binds them to fight anyone who withholds the Silmarils from them. This terrible oath results in much future trouble including mass murder and the war of Elf against Elf. Fëanor leads many of the Noldor back to Middle-earth. His flight, during the First Age of Middle-earth, leads to unending grief for the Elves and eventually for the Men of Middle-earth. Five major battles are fought in Beleriand, but ultimately the Noldor and all the people who took the oath fail in their attempt to regain the Silmarils from Morgoth.

One of the Silmarils is recovered by Beren and Lúthien through great peril and loss, when Lúthien sends Morgoth to sleep with her singing and Beren cuts it from his crown. The werewolf Carcharoth attacks them as they leave Angband and swallows Beren's hand containing the Silmaril; this drives Carcharoth mad. He is killed by Huan the Hound, who dies from his wounds, and the Elf-captain Mablung cuts the Silmaril out. It is taken to the Valar in the West by Eärendil, heir of Beren and Lúthien, as a token of repentance for Fëanor's rebellion. The Valar then set this Silmaril as a star in the sky: it is fixed to the mast of Eärendil's ship, forever sailing across the sky. The other two gems remain in Morgoth's hands, until they are taken from him by a servant of the Vala Manwë at the end of the War of Wrath. However, soon afterwards, they are stolen by Fëanor's two remaining sons, Maedhros and Maglor, as they try to fulfil the oath they had sworn so many years before. But the jewels burn their hands, in denial of their rights of possession, as they had burned Morgoth's hands before. In agony, Maedhros throws himself and his Silmaril into a fiery pit, and Maglor throws his Silmaril into the sea. Thus the Silmarils remain in the sea, the earth, and the sky—their light present but inaccessible to those in Middle-earth.

According to a prophecy of Mandos, following Melkor's final return and defeat in the Dagor Dagorath (Battle of Battles), the world will be changed and the Silmarils will be recovered by the Valar. Then Fëanor will be released from the Halls of Mandos, and he will give Yavanna the Silmarils. She will break them, and with their light she will revive the Two Trees. The Pelóri Mountains will be flattened, and the light of the Two Trees will fill the world in eternal bliss.

The idea of the Silmaril is connected to Tolkien's philological exploration of the Old English word Siġelwara, which was used in the Old English Codex Junius to mean "Aethiopian". Tolkien wondered why the Anglo-Saxons should have had a word with this meaning, and conjectured that it had once meant something else, which he explored in his essay "Sigelwara Land".