The fictional forest inhabited by Ents in J.R.R. Tolkien’s The Lord of the Rings was Fangorn Forest.
Fangorn lies at the eastern end of the Misty Mountains, near Rohan and Isengard. It is the home of Treebeard and other Ents, ancient tree-like beings who remember a much older history of Middle-earth. Merry and Pippin encounter Treebeard after escaping from the Orcs who captured them.
The Ents’ gathering, known as the Entmoot, leads to their attack on Isengard. Their action becomes one of the major turning points in the War of the Ring, as Saruman’s industrial destruction of the surrounding countryside is answered by the beings who protect the forest.
Fangorn is distinct from Mirkwood, the great woodland farther east, and from the Old Forest near the Shire. Lothlórien is another important woodland, associated with Galadriel and Celeborn, but it is not the home of the Ents in the story.