The Battle of Helm's Deep, also called the Battle of the Hornburg, is a fictional battle in J. R. R. Tolkien's The Lord of the Rings that saw the total destruction of the forces of the Wizard Saruman by the army of Rohan, assisted by a forest of tree-like Huorns.

Helm's Deep was a valley in the north-western White Mountains of Middle-earth. Helm's Deep, with its fortress the Hornburg, becomes the refuge of some of the army of Rohan, the Rohirrim, under King Théoden, from assault by the forces of Saruman. Although Théoden says that "the Hornburg has never fallen to assault," in the battle a massive army of Uruk-hai and Dunlendings sent by Saruman almost overwhelms the defences. Saruman's Orcs breach the fortress wall that blocks the valley by setting off an explosion in a culvert; Aragorn names it "Saruman's devilry" and "the fire of Orthanc"; the critic Tom Shippey calls it "a kind of gunpowder". The defenders hold out in the fortress until dawn, when Théoden and Aragorn lead a cavalry charge that drives the Orcs from the fortress. They are surprised to see the valley to the enemy's rear blocked by a forest of tree-like Huorns that have walked from Fangorn in the night. On the side of the valley are relieving forces assembled by Gandalf and Erkenbrand, a Rohirrim leader. These attack, driving the Orcs into the angry Huorn forest, from which the Orcs never emerge; the Huorns bury the Orcs' bodies in an earthen mound known as "Death's Down".

Tolkien based Helm's Deep on England's Cheddar Gorge, and the Glittering Caves of Aglarond on the cave complex that he had visited there. The army of Rohan was according to Tolkien armed and equipped much like that of the armies depicted in the Bayeux Tapestry. He noted further that his walking forest was partly a response to Shakespeare's Macbeth, which tells of the coming of "Great Birnam Wood to high Dunsinane hill". Scholars have likened the way Aragorn, Éomer, and Gimli heroically hold off the army of Orcs to Horatius Cocles's heroic defence of a bridge of ancient Rome.

Peter Jackson's 2002 film The Two Towers makes the battle dramatic, following Tolkien's account quite closely, but with changes to the forces involved: the defenders include a group of Elf-warriors sent by Elrond; the attackers include neither men nor wargs (battle-wolves).

Helm's Deep is based on the Cheddar Gorge, a limestone gorge 400 ft (120 m) deep in the Mendip Hills, with a large cave complex that Tolkien visited on his honeymoon in 1916 and revisited in 1940, and which he acknowledged as the origin of the Glittering Caves of Aglarond at the head of Helm's Deep, behind the fortress.

Helm's Deep is properly the narrow gorge or ravine at the head of a larger valley (the Deeping-coomb), but the name is also used for the fortifications at the mouth of the gorge and the larger valley below. The gorge, which wound deep into the White Mountains at the feet of the Thrihyrne mountain, led into the Glittering Caves of Aglarond, an extensive series of spectacular speleothems. In The Lord of the Rings, the Dwarf Gimli, who like all dwarves is well versed in geology, is horrified that the caves are used only as a refuge, describing them lyrically as:

immeasurable halls, filled with everlasting music of water that tinkles into pools, as fair as Kheled-zâram in the starlight. […] when torches are kindled and men walk on the sandy floors under the echoing domes, ah! then […] gems and crystals and veins of precious ore glint in the polished walls; and the light glows through folded marbles, shell-like, translucent as the living hands of Queen Galadriel. There are columns of white and saffron and dawn-rose […] fluted and twisted into dreamlike forms; they spring up from many-coloured floors to meet the glistening pendants of the roof: wings, ropes, curtains fine as frozen clouds; spears, banners, pinnacles of suspended palaces! Still lakes mirror them: a glimmering world looks up from dark pools covered with clear glass; cities such as the mind of Durin could scarce have imagined in his sleep, stretch on through avenues and pillared courts, or into the dark recesses where no light can come.

The mouth of the gorge, Helm's Gate, was closed by the battlemented Deeping Wall, 20 ft (6.1 m) tall, and wide enough for four men to stand abreast, with a culvert for the Deeping-stream which flowed down the valley. At one end of the wall the Hornburg castle stood on a spur of the mountain; a long stair led to its rear gate, and a long causeway led down forwards from its main gate. About two furlongs (400 metres) down from the gate was an outer trench and rampart, Helm's Dike, built right across the Deeping-coomb. Tolkien drew detailed sketches of the fortifications.