In Norse mythology, Fenrir or Fenrisulfr is a wolf, the son of Loki and the giantess Angrboða. Fenrir is bound by the gods, but is ultimately destined to grow too large for his bonds and swallow Odin whole during the course of Ragnarök. He will be slain by Odin's son, Viðarr, who will slice his belly open, avenging his father.

Fenrir has two sons, Hati and Skoll. Skoll chases the horses, that drag the chariot which contains the sun. Hati chases the moon. 'Skoll', in certain circumstances, is used as a heiti to refer indirectly to the father (Fenrir) and not the son. This ambiguity works in the other direction also, for example in the Vafþrúðnismál, where a confusion exists in stanza 46 where Fenrir is given the sun-chasing attributes of his son Skoll. This can mostly be accounted for by the use of Hróðvitnir and Hróðvitnirson to refer to both Fenrir and his sons.

Learning from the prophecy of the sybil (cf. Völuspá ) and from his contest with Vafþrúðnir (related in Vafþrúðnismál) that the children of Loki and Angrboða would bring trouble to the gods, Odin had the wolf brought to him along with his brother Jörmungandr and his sister Hel.

After casting Jörmungandr into the sea and Hel down into the land of the dead, Odin had the wolf raised among the Æsir. Only the god Týr was daring enough to feed the growing monster. The gods, urged by the wolf's increasing strength and by prophecies that he would be their destruction, attempted to bind the great beast. Twice he agreed to be chained and twice easily burst out of two successive fetters. The first, made of iron, was called Lœðingr. The second, also of iron, but of twice the strength, was called Drómi.

Odin then had the dwarfs forge the chain Gleipnir ("deceiver" or "entangler"). It appeared to be only a silken ribbon but was made of six wondrous ingredients: the sound of a cat's footfall, the beard of a woman, the roots of a mountain, bear's sinews (meaning nerves, sensibility), fish's breath, and bird's spittle. Skírnir, Freyr's messenger, brought it back to Ásgarðr.

Then, in the island called Lyngvi ("Heathery") in the lake called Ámsvartnir ("Red-black") (places unknown to us), the gods challenged Fenrisulfr to break this chain also. But the wolf noted the thinness and fineness of construction of Gleipnir and not unreasonably suspected a trick. He agreed to make the test only if one of the gods was willing to place his hand in the wolf's mouth during the binding as a pledge to free him if he failed to break the chain. No god was willing to do this, until Týr stood forth and placed his hand in the wolf's mouth. Fenrisulfr strained to burst the chain but the more he struggled the tighter he was held. When the gods would not free him, the wolf bit off Týr's hand at the wrist, the point afterwards called "the wolf joint".

It is prophesied that at Ragnarök the wolf will at last break free and join forces with the enemies of the gods and will then swallow Odin himself whole. After that Viðarr, Odin's son, will slay the wolf to avenge his father.