Denethor is the name of two fictional characters from J. R. R. Tolkien's universe of Middle-earth.

Denethor was the last Steward of Gondor in the third age of Middle-earth and the father of Boromir and Faramir. He is known to have used a palantír to communicate with Sauron and probe his strength. The effort aged him quickly, and the knowledge of Sauron's overwhelming force depressed him greatly. The death of Boromir, his firstborn and favorite, together with the siege and apparent doom of his capital city, drove him over the edge into insanity. He ordered his men to burn him alive on a funeral pyre with the palantír in his hands. He also tried to take the grievously injured and apparently dying Faramir with him, but was thwarted in that by the timely intervention of Peregrin Took. In Peter Jackson's films of the books, Denethor was played by John Noble.

An earlier Denethor was the son of the Nandorin Elf leader Lenw (or Dan, who lived during the Years of the Trees. Hearing of the power of Thingol, he led many of his people over the Ered Luin to Beleriand. They settled in Ossiriand, renaming it Lindon, in the east of Beleriand, and Denethor became their king. They were called the Laiquendi, or Green-elves. Some time later, Morgoth loosed an army on Beleriand. The lightly-armed Laiquendi were driven back to the great hill of Amon Ereb, where Denethor was slain. The Laiquendi became a reclusive people, seldom taking up arms, and never again naming a king.