Mirkwood was a great wood east of the Misty Mountains in Rhovanion, in J. R. R. Tolkien's fictional Middle-earth.

Mirkwood was the forest dwelling of a Silvan Elven kingdom, ruled by King Thranduil. It had been called Greenwood the Great until around the year 1050 in the Second Age of the Years of the Sun, when a shadow of the dark lord Sauron fell upon it, and men began to call it Mirkwood, or Taur-nu-Fuin and Taur-e-Ndaedelos in the Sindarin tongue. Sauron established himself at the hill-fortress of Dol Guldur on Amon Lanc, and drove Thranduil and his people ever northward, so that by the end of the Third Age they were a diminished and wary people, who had entrenched themselves beyond the Mountains of Mirkwood (Emyn Fuin, formerly the Emyn Duir or "Dark Mountains"). The Old Forest Road or Old Dwarf Road crossed the forest east to west, but due to its relative proximity to Dol Guldur, the road was mostly unusable. The Elves made a path farther to the north, which ended somewhere in the marshes south of the Long Lake of Esgaroth.

In The Hobbit, Bilbo and the dwarves came across many great spiders, the breed of Shelob. Shortly after the Dwarves's escape, they were captured by the Elves. After or during these events the White Council attacked Dol Guldur, and Sauron fled to Mordor. Mirkwood became less evil after that for a while.

Gollum, after his release from Mordor, was captured by Aragorn and brought a prisoner to Thranduil's halls. He escaped during an Orc raid, and fled south to Moria.

After Sauron was reduced to a powerless "spirit of malice" at the conclusion of the Third Age, the darkness was lifted from Mirkwood, and it became known again by its old name of Eryn Lasgalen, Sindarin for the Wood of Greenleaves.

The name "Mirkwood" was not invented by Tolkien — it originates in old Norse and German folklore, wherein it was used to refer to a number of anonymous magical forests. It has been speculated that the Black Forest of Germany derived its name from this title.