In J. R. R. Tolkien's fictional universe of Middle-earth, Isengard, a translation of the Sindarin Angrenost, was a large fortress. Both names mean "Iron Fortress".

Isengard was built in the Second Age around the tower of Orthanc by the Númenóreans in exile. Its location was at the north-western corner of the Southern Kingdom of Gondor, guarding the Fords of Isen from enemy incursions into Calenardhon together with the fortress of Aglarond to its south.

The river Angren (or Isen) began at Mount Methedras behind Isengard, which also formed its northern wall. The other three sides were guarded by a large wall, known as the Ring of Isengard, which was only breached by the inflow of the river Angren at the north-east through a portcullis, and the gate of Isengard at the south, at both shores of the river.

Isengard was a green and pleasant place, with many large trees and grass fields, fed by the Angren. Orthanc stood in the exact centre.

During the early Third Age the land of Calenardhon became depopulated, and the last warden of Orthanc was recalled to Minas Tirith. Isengard remained guarded by a small company, led by a hereditary captain, much like happened in Angrenost at the time.

After Calenardhon was given to the Éothéod by Cirion, Steward of Gondor, and became Rohan, Isengard remained part of Gondor, although Gondor almost forgot about it. The small guard intermarried much with the Dunlendings, until the fortress became a Dunlending fortress in all but name. Orthanc however remained closed, as the Steward of Gondor alone held the keys.

During the rule of Rohan's King Déor it turned out that Isengard had become openly hostile to the Rohirrim. Using Isengard as their base, the Dunlendings continued to raid Rohan during Déor's son Gram's rule, until during the rule of Gram's son Helm Hammerhand a Dunlending lord, Freca and his son Wulf nearly managed to destroy the Rohirrim. The Rohirrim eventually won, taking Isengard, and guarding it for Gondor.

During the time of the Steward of Gondor Beren, Saruman the White suddenly reappeared from the East, and he offered to guard Isengard. Beren gladly gave him the keys to Orthanc, and Saruman settled there. After him the valley became known as Nan Curunír, or "Valley of Saruman".

During the War of the Ring Isengard was Saruman's base of operations against the Rohirrim, and he defiled the valley, cutting down its trees and damming and diverting the Angren. Isengard's valley was destroyed by deep pits, used for breeding Uruk-hai and smithying weapons. Isengard became home to countless Orcs, whom Saruman used to try to conquer Rohan. Eventually an army of Ents and Huorns led by Treebeard of Fangorn attacked Isengard, taking the fortress, although they could not take Orthanc.

The Hobbits Meriadoc Brandybuck and Peregrin Took as the new "doorwardens" received Théoden King of Rohan, Aragorn and Gandalf at the gates. Saruman was confronted but refused to beg for pardon, and he was released into Treebeard's guard together with his servant Gríma. Saruman abandoned Isengard afterwards.

During the Fourth Age Isengard was restored, and Treebeard replanted many trees. Orthanc became again a tower of the Reunited Kingdom of Aragorn King Elessar.

Isengard is also the name of two Tolkien Metal bands, see Isengard (band).