In Germanic heroic legend and folklore, Fáfnir, was a dwarf or other type of humanoid, who had shifted into the hamr of a Germanic dragon (a worm, "serpent", in period nomenclature), in order to protect a treasure, and then was slain by a member of the Völsung family, typically Sigurð.

In Nordic mythology, he is the son of Hreiðmarr, and brother of Regin and Ótr. He is attested throughout the Völsung Cycle, where, Fáfnir commits patricide out of greed, taking the ring and hoard of the dwarf Andvari, and shapeshifting into a dragon. Fáfnir's brother Regin later assisted Sigurð in obtaining the sword Gram, by which Fáfnir is killed. Fáfnir has been identified with an unnamed dragon killed by a Völsung in other Germanic works, including Beowulf, the Nibelunglied and a number of skaldic poems. Fáfnir and his killing by Sigurð are further represented in numerous medieval carvings from the British Isles and Scandinavia, and a single axe head in a Scandinavian style found in Russia. The story of Fáfnir has continued to have influence in the modern period, such as in the works of J.R.R Tolkien, who drew inspiration from the tale of Fáfnir in his portrayals of Smaug and Gollum.

The name "Fáfnir" has been translated from Old Norse as "the embracer", lit. 'the fathomer' (archaised: "fathomner"), stemming from the verb faðma ("fathom, embrace") with the inchoative verb-suffix -na and agent noun-suffix -ir. The spelling fáfn- stems from an excrescence following the m, introducing a short /b/ or /p/, in turn morphing into an /f/, thus something akin to /faðmnair/ → /faðmb͡hnair/ → /fahmb͡hnir/ → /fámhp͡hnir/ → /fáfnir/ (compare Danish: favne, Old Swedish: fambna, fampna, from Old Norse: faðma). Cognates of faðmr (the noun of faðma), includes Old Saxon: fathmōs, Middle Low German: vadem, Dutch: vadem, vaam, Old Frisian: fethm, Old High German: fadam, fadum, German: faden, Old English: fæðm and English: fathom.

The name can be interpreted in various ways. It could potentially be a play on Fáfnir's worm-form. The root sense, "the one who embraces", could refer to the worm coiling around the treasure to brood over it, but a double entendre could also be "the one of many fathoms", referencing the measurement fathom, relating to the worm's massive length.

Before taking the form of a dragon, Fáfnir's appearance is not explicitly described. It has been noted though that his brother, Regin, is described as being a dwarf in Norna-Gests þáttr and resembling a dwarf (Old Norse: dvergr of vǫxt), however, it is neither clear if relatives of dwarfs are also dwarfs, nor how dwarfs were conceived of as appearing. It has been noted that dwarf appearances were presented in the medieval period as highly diverse and they could even change their form, as with Andvari and potentially also Fáfnir.

While in the form of a worm in Fáfnismál, Fáfnir is described as flightless and snake-like while at the same point in the narrative of the later Völsunga saga, he has shoulders, suggesting legs, wings or both. This change is consistent with the wider trend in Germanic portrayals of dragons, and likely results from influence from continental Europe that was facilitated by Christianisation and the increased availability of translated romances.

Fáfnir's tale is attested in multiple medieval accounts which refer to a shared story with a general structure as follows:

The Poetic Edda contains two poems that mention Fáfnir. In the prose of the first, Reginsmál, the eponymous figure Regin tells Sigurð that once Odin, Loki, and Hœnir went to the foss of the dwarf Andvari who lived there in the form of a pike. In that water also lived Fáfnir and Regin's brother, Ótr who was eating a salmon on the bank, in the shape of an otter. Loki killed Ótr with a stone and the gods skinned him before seeking lodgings at the house of Hreiðmarr, the father of Fáfnir and his siblings. Seeing the otter skin, the family seized the gods and demanded that the gods fill and cover the skin in red gold as weregild. Loki caught Andvari using Rán's net and exports the gold from him. In the poem, Andvari curses the gold which the gods give to Hreiðmarr. Fáfnir and Regin asked their father for some of the weregild but he refused, leading Fáfnir to kill him. As he dies, Hreiðmarr calls out to his daughters Lyngheiðr and Lofnheiðr, but they take no action. Fáfnir then takes all the hoard, refusing to share it with his brother, and takes to Gnitaheath, where he took the shape of a worm. Along with Andvari's ring, Andvaranaut, the treasure here is described as including the Helm of fear (Old Norse: ægishjálmr).