In J. R. R. Tolkien's legendarium, Lothlórien or Lórien is the fairest realm of the Elves remaining in Middle-earth during the Third Age. It is ruled by Galadriel and Celeborn from their city of tree houses at Caras Galadhon. The wood-elves of the realm are called Galadhrim.

The realm, a broad woodland between the Misty Mountains and the River Anduin, is the Elven centre of resistance against the Dark Lord Sauron in The Lord of the Rings. Galadriel had one of the Three Elf-Rings, and used it to keep Sauron from seeing into Lothlórien. The Company of the Ring spent some time in Lothlórien after passing through Moria. Galadriel prepared them for their quest with individual gifts.

Scholars have noted that Lothlórien represents variously an Earthly Paradise; an Elfland where time is different, reflecting the traditions of European folklore; and a land of light striving biblically with the darkness of evil.

Tolkien gave the forest many different names, reflecting its fictional history and the way it is perceived by the different peoples of Middle-earth.

Early in the First Age, some of the Eldar left the Great March to Valinor and settled in the lands east of the Misty Mountains. These elves became known as the Nandor, and later as the Silvan Elves. Galadriel made contact with an existing Nandorin realm, Lindórinand, in what became Lothlórien, and planted there the golden mallorn trees which Gil-galad had received as a gift from Tar-Aldarion.

The culture and knowledge of the Silvan elves was enriched by the arrival of Sindarin Elves from west of the Misty Mountains, and the Silvan language was gradually replaced by Sindarin. Amongst these arrivals was Amdír, who became their first lord, as well as Galadriel and Celeborn, who fled the destruction of Eregion during the War of the Elves and Sauron. In the Third Age, Amroth, the former Lord of Lothlórien, went to the south of Middle-earth with his beloved Nimrodel, but drowned in the Bay of Belfalas after she went missing in the Ered Nimrais and never returned. Control of Lothlórien passed to Galadriel and Celeborn. Galadriel's Ring of Power preserved the land from death and decay, and warded off Sauron's gaze.

As the War of the Ring loomed, the Company of the Ring, emerging from the dark tunnels of Moria and seeing their leader Gandalf perish, was brought through Lothlórien to Caras Galadhon, and there met the Lord and Lady of the Galadhrim. The Fellowship spent roughly a month in Lothlórien, though it seemed to them only a few days. Before they left, Galadriel allowed Samwise and Frodo to look in the Mirror of Galadriel, giving them a glimpse of events in the future or at other times; she also tested the loyalty of Fellowship members, and gave each of them a gift for their quest.

After the fall of Sauron, Galadriel and Celeborn rid Dol Guldur of Sauron's influence. Galadriel left for Valinor at the beginning of the Fourth Age, and Celeborn later followed her. The city slowly became depopulated and Lothlórien faded. By the time of the death of Queen Arwen, Celeborn and Galadriel's granddaughter, Lothlórien itself was deserted.