Fenrir (Old Norse 'fen-dweller') or Fenrisúlfr (Old Norse "Fenrir's wolf", often translated "Fenris-wolf"), also referred to as Hróðvitnir (Old Norse "fame-wolf") and Vánagandr (Old Norse 'monster of the [River] Ván'), is a monstrous wolf in Norse mythology. In Old Norse texts, Fenrir plays a key role during the events of Ragnarök, where he is foretold to assist in setting the world aflame, resulting in the collapse of humanity and society, and killing the god Odin.

Fenrir, along with Hel and Jörmungandr, is a child of Loki and female jötunn Angrboða. He is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda and Heimskringla, composed in the 13th century. In both the Poetic Edda and Prose Edda, Fenrir is the father of the wolves Sköll and Hati Hróðvitnisson, is a son of Loki and is foretold to kill the god Odin during the events of Ragnarök, but will in turn be killed by Odin's son Víðarr.

In the Prose Edda, additional information is given about Fenrir, including that, due to the gods' knowledge of prophecies foretelling great trouble from Fenrir and his rapid growth, the gods bound him and as a result Fenrir bit off the right hand of the god Týr. Depictions of Fenrir have been identified on various objects and scholarly theories have been proposed regarding Fenrir's relation to other canine beings in Norse mythology. Fenrir has been the subject of artistic depictions and he appears in literature.

Fenrir is mentioned in three stanzas of the poem Völuspá and in two stanzas of the poem Vafþrúðnismál. In stanza 40 of the poem Völuspá, a völva divulges to Odin that, in the east, an old woman sat in the forest Járnviðr "and bred there the broods of Fenrir. There will come from them all one of that number to be a moon-snatcher in troll's skin." Further into the poem the völva foretells that Odin will be consumed by Fenrir at Ragnarök:

Then is fulfilled Hlín's
second sorrow,
when Óðinn goes
to fight with the wolf,
and Beli's slayer,
bright, against Surtr.
Then shall Frigg's
sweet friend fall.

In the stanza that follows the völva describes that Odin's "tall child of Triumph's Sire" (Odin's son Víðarr) will then come to "strike at the beast of slaughter" and with his hands he will drive a sword into the heart of "Hveðrungr's son", avenging the death of his father.

In the first of two stanzas mentioning Fenrir in Vafþrúðnismál Odin poses a question to the wise jötunn Vafþrúðnir:

Much I have travelled, much have I tried out,
much have I tested the Powers;
from where will a sun come into the smooth heaven
when Fenrir has assailed this one?