In J.R.R. Tolkien's Middle-earth legendarium, Théoden was the seventeenth King of Rohan, and last of the Second Line.

Théoden was the oldest son of Thengel, and became king after the death of his father in T.A. 2980. Théoden spoke Sindarin and Westron rather than Rohirric, and in his youth spent time in Gondor.

His sister Théodwyn lived with him in Edoras, and after she and her husband both died he adopted her children Éomer and Éowyn as his own. He had a son Théodred, whose mother Elfhild died in childbirth.

By the time of the War of the Ring Théoden had been king for nearly 30 years, and was getting old and tired. He was increasingly misled by his chief advisor Gríma (or Wormtongue as most others in the Mark called him), who was secretly in the employ of Saruman. Gríma may even have been poisoning his lord.

In the last years before the War Théoden let his rule slip out of his hands, and Gríma became increasingly powerful. Rohan was troubled again by Orcs and Dunlendings, who operated under the will of Saruman, ruling from Isengard.

When Théoden's son Théodred was mortally wounded at a battle at the Fords of Isen in battle with the Orcs of Saruman, his nephew Éomer became his heir. Éomer was out of favor with Wormtongue and was eventually arrested.

When Gandalf and Aragorn appeared before him, Théoden initially rebuffed Gandalf's advice to ride out against Saruman, but after being healed by him he restored his nephew, took up his sword, and led the Riders of Rohan into battle at Helm's Deep. After this he became known as Théoden Ednew, the Renewed, because he had thrown off the yoke of Saruman.

He led the Rohirrim to the aid of Gondor at 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 frightened by the Ringwraith's fell beast. He was immediately avenged by his niece Éowyn and the Hobbit Meriadoc Brandybuck, both of whom had ridden to war in secret.

In Tolkien's fictional etymology, the name Théoden is a translation of the original Rohirric Tûrac, an old word for King showing influence from the Elvish stem tur-, power/mastery, also present in Turgon and related names.

The name Théoden is probably taken from the Anglo-Saxon word "þeoden", meaning "lord". It is related to the Old Norse word þjóðann, meaning "leader of the people" (i.e. "King"). Théoden's sword was called Herugrim.

In the 1981 BBC Radio 4 version of The Lord of the Rings, Théoden's death is described in song rather than dramatized conventionally, which tends to lessen its impact.

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. He then goes to Helm's Deep to take his people to safety rather than to make a stand against the enemy. The movie also has Théoden aware of Éowyn's presence at his death, whereas in the book he says his farewells to Merry and does not know that Éowyn is also there.