In the fiction of J. R. R. Tolkien, Théoden was king of Rohan in Middle-earth at the end of the Third Age, and played a key role in the events of The Lord of the Rings.
The uncle of Éowyn and Éomer, Théoden was misled by Gríma (or Wormtongue as most others in the Mark called him). Théoden initially rebuffed Gandalf's advice, but after being healed by him he led the Riders of Rohan into battle at the Helm's Deep and the Battle of the Pelennor Fields.
In that battle he challenged the Witch-King of the Nine Ringwraiths, and died when his horse Snowmane fell upon him after being spooked by the Ringwraith's flying beast. He was immediately avenged by his niece Éowyn and the Hobbit Meriadoc Brandybuck, both of whom had ridden to war in secret.
The name Théoden seems to be derived from the Old Norse word 'þjóðann' meaning "king".
Peter Jackson's film The Lord of the Rings: The Two Towers (2002) deviates from Tolkien's story by having Théoden (played by Bernard Hill) actually possessed by Saruman rather than simply deceived by Gríma.