Middle-earth is the name used for the inhabitable parts of J.R.R. Tolkien's fictional Arda (ancient Earth) where the (canonical) stories in his legendarium take place. "Middle-earth" is a literal translation of the Old English term middangeard, referring to this world, the habitable lands of men. Tolkien translated "Middle-earth" as Endor (or sometimes Endóre) and Ennor in the Elvish languages Quenya and Sindarin, respectively. Mythologically, the north of Endor became the Eurasian land-mass after the primitive Earth was transformed into the round world of today. Less formally, the term "Middle-earth" is also often used to refer to the entire setting of Tolkien's legendarium.

Middle-earth's setting is in a fictional period in Earth's own past. Tolkien insisted that Middle-earth is (part of) our Earth in several of his letters, in one of them (no. 211) estimating the end of the Third Age to about 6,000 years before his own time. The action of the books is largely confined to the north-west of the Endor continent, implicitly corresponding to modern-day Europe. The history of Middle-earth is divided into several Ages: The Hobbit and the main text of The Lord of the Rings deal exclusively with events toward the end of the Third Age and conclude at the dawn of the Fourth Age, while The Silmarillion deals mainly with the First Age. The world (Arda) was originally flat but was made round near the end of the Second Age by Eru Ilúvatar, the Creator.

Much of the knowledge of Middle-earth is based on writings that Tolkien did not finish for publication during his lifetime. This has caused some controversy over what is considered "canonical"; for more information, see Middle-earth canon.