In J. R. R. Tolkien’s fictional universe of Middle-earth, the Old Forest was a daunting and ancient woodland just beyond the eastern borders of the Shire. Its first and main appearance in print was in the chapter of the 1954 The Fellowship of the Ring titled "The Old Forest". The hobbits of the Shire found the forest hostile and dangerous; the nearest, the Bucklanders, planted a great hedge to border the forest and cleared a strip of land next to it. A malign tree-spirit, Old Man Willow, grew beside the River Withywindle in the centre of the forest, controlling most of it, though Tom Bombadil presides over the forest.

The scholar Verlyn Flieger has observed that the hostility of the Old Forest and of Old Man Willow contradicts Tolkien's otherwise protective stance for wild nature. Scholars have discussed the symbolism of the Old Forest, likening it to "Old England", and, given that the protagonist Frodo Baggins calls it "the shadowed land", to Death.

The Old Forest lay near the centre of Eriador, a large region of north-west Middle-earth. It was one of the few survivors of the primordial forests which had covered much of Eriador before the Second Age. Indeed, it had once been but the northern edge of one immense forest which reached all the way to Fangorn forest, hundreds of miles to the south-east.

The vicinity of the Old Forest was the domain of three nature-spirits: Tom Bombadil, Goldberry, and Old Man Willow. The powers of these beings doubtless contributed to its survival when other forests were destroyed.

Old Man Willow, along with the Barrow-wight and Tom Bombadil himself, first appeared in Tolkien's narrative poem The Adventures of Tom Bombadil, where Old Man Willow trapped Bombadil himself briefly. Willow is portrayed as a sentient and evil willow tree with powers including hypnosis and the ability to move his roots and trunk. Some characters of the story speculate that he may have been related to the Ents, or possibly the Huorns. However, unlike Ents or Huorns, he is portrayed more like a tree, with roots in the ground, and without the ability to move from place to place. Tom Bombadil had power over Old Man Willow, and checked the evil as much as he could, or was willing.

According to Tom Bombadil, at the dawn of time, long before even the Awakening of the Elves, trees were the only inhabitants of vast stretches of the world. Because the Elves awoke far in the East, it was still a considerable time before any other beings spread into the vast primeval forests of western Middle-earth. A handful of trees survived from this time until the present day, who are angered at the encroachment of Elves and Men and their dominion over the earth; trees who bitterly remember a time long ago when they were as Lords of vast regions of the world. Bombadil relates that of the corrupted trees of the Old Forest, "none were more dangerous than the Great Willow; his heart was rotten, but his strength was green; and he was cunning, and a master of winds, and his song and thought ran through the woods on both sides of the river. His grey thirsty spirit drew power out of the earth and spread like fine root-threads in the ground, and invisible twig-fingers in the air, till it had under its dominion nearly all the trees of the Forest from the Hedge to the Downs."

In the First Age, Tom Bombadil "was here before the river and the trees". In the Spring of Arda, plants emerge, possibly including Old Man Willow. In the Years of the Trees, Elves skirted the forest on their primeval migration to Beleriand and the West; they were observed by Bombadil.

By the time Sauron had been defeated and driven from Eriador, most of its old forests had already been destroyed, leaving remnants such as the Old Forest. (Other vestiges included Woody End in the Shire, Chetwood in Bree-land, and Eryn Vorn in Minhiriath.) The Old Forest was now "hostile to two legged creatures because of the memory of many injuries."