A jötunn (also jotun; plural jötnar; in the normalised scholarly spelling of Old Norse, jǫtunn /ˈjɔːtʊn/; or, in Old English, eoten, plural eotenas) is a type of being in Germanic mythology. In Norse mythology, jötnar are often contrasted with gods (the Æsir and Vanir) and with other non-human figures, such as dwarfs and elves, although the groupings are not always mutually exclusive. The entities included in the jötunn category are referred to by several other terms, including risi, þurs (or thurs) and troll if male and gýgr or tröllkona if female. The jötnar typically dwell across boundaries from the gods and humans in lands such as Jötunheimr.

The jötnar are frequently attested throughout the Old Norse records, with eotenas also featuring in the Old English epic poem Beowulf. The usage of the terms is dynamic, with an overall trend that the beings become portrayed as less impressive and more negative as Christianity becomes more influential over time. Although the term "giant" is sometimes used to gloss the word "jötunn" and its apparent synonyms in some translations and academic texts, this is seen as problematic by some scholars as jötnar are not necessarily notably large.

The terms for the beings also have cognates in later folklore such as the English yotun, Danish jætte and Finnish jätti which can share some common features –such as being turned to stone in the day and living on the periphery of society.

Old Norse jötunn and Old English eoten developed from the Proto-Germanic masculine noun *etunaz. Philologist Vladimir Orel says that semantic connections between *etunaz and Proto-Germanic *etanan ('to eat') make a relation between the two words likely. The words are cognate with ettin, an archaic word for a type of being. Old Norse risi and Old High German riso derive from the Proto-Germanic masculine noun *wrisjon. Orel observes that the Old Saxon adjective wrisi-līk 'enormous' is likely also connected. Old Norse þurs, Old English þyrs, and Old High German duris 'devil, evil spirit' derive from the Proto-Germanic masculine noun *þur(i)saz, itself derived from Proto-Germanic *þurēnan, which is etymologically connected to Sanskrit turá- 'strong, powerful, rich'. Several terms are used specifically to refer to female entities that fall into this wider category, including íviðja (plural íviðjur), gýgr (plural gýgjar) and tröllkona (plural tröllkonur).

Terms for jötnar are also found in Old Norse compound words such as bergrisi, ("mountain-risi") and hrímþurs ("rime-þurs", or "frost-þurs").

The cognates jötunn and eoten, and þurs and þyrs have been equated by scholars such as J. R. R. Tolkien and Rudolf Simek, with the words being used to describe the being in either Old Norse or Old English respectively.

In the Eddas, jötnar are beings typically with similar power to the gods and may also be referred to by the negative terms troll and þurs. The harmful nature of þursar is also described in the Icelandic and Norwegian rune poems, where they are identified for causing strife to women. Descriptions of the appearance of jötnar are uncommon; however, the progenitor of the jötnar is described as having the form of a man. Some female jötnar are described as being beautiful, such as Gerðr and Hymir's partner, while others are described as monstrous and having many heads. Some dwarfs are described as jötnar such as Regin and Fáfnir, while in Alvíssmál, the eponymous dwarf is noted for having the likeness of a þurs.

As the influence of Christianity grew, jötnar became demonised and typically portrayed as less intelligent, easier to outwit and more monstrous, as is common with giants in later Germanic folklore. In some later sagas, such as Bárðar saga Snæfellsáss, risar are clearly distinct from jötnar however in others the terms are used interchangeably, albeit with an overall trend that jötnar have begun to be seen negatively relative to risar.