Mirkwood, that received its ominous name in the Third Age, was the greatest forest of Middle-earth, located in the eastern region of Rhovanion between the Grey Mountains and Calenardhon. Before the end of the first millennium of the Third Age, it was also known as Greenwood the Great, Eryn Galen, or Taur-e-Ndaedelos, and at the end of the Third Age after the destruction of the One Ring, it was renamed Eryn Lasgalen, the Wood of Greenleaves.

Mirkwood was a dense and heavy woodland that made up much of the eastern portion of Rhovanion or Wilderland, that maintained its borders and relative shape for many ages. Its natural land features included (in the northern part of the forest) the Mountains of Mirkwood, a sizable river referred to in J.R.R. Tolkien's map as the Forest River, that ran from the Grey Mountains down to Long Lake, and a smaller river that ran from the Mountains of Mirkwood to join with the Forest River west of the Elvenking's Halls. This smaller river was enchanted (or polluted) to such an extent that it caused slumber and forgetfulness to anyone who fell into it. Mirkwood's climate was relatively mild. Except for ways through the thickets of the forest, there were very few common routes through Mirkwood save for the Old Forest Road and the Forest Path. Mirkwood was approximately 600 miles long from north to south, and 250 miles across at its width. During the events of The Hobbit it was home to giant spiders, and the Woodland Realm of King Thranduil and his Wood-elves; the Woodmen of Mirkwood also inhabited a small part of the forest.