Mjölnir (UK: /ˈmjɒlnɪər/ MYOL-neer, US: /ˈmjɔːlnɪər/ MYAWL-neer; from Old Norse Mjǫllnir [ˈmjɔlːnir]) is the hammer of the thunder god Thor in Norse mythology, used both as a devastating weapon and as a divine instrument to provide blessings. The hammer is attested in numerous sources, including the 11th century runic Kvinneby amulet, the Poetic Edda, a collection of eddic poetry compiled in the 13th century, and the Prose Edda, a collection of prose and poetry compiled in the 13th century. The hammer was commonly worn as a pendant during the Viking Age in the Scandinavian cultural sphere, and Thor and his hammer occur depicted on a variety of objects from the archaeological record. Today the symbol appears in a wide variety of media and is again worn as a pendant by various groups, including adherents of modern Heathenry.

The etymology of the hammer's name, Mjǫllnir, is disputed among historical linguists. Old Norse Mjǫllnir developed from Proto-Norse *melluniaR and one proposed derivation connects this form to Old Church Slavonic mlunuji and Russian molnija meaning 'lightning' (either borrowed from a Slavic source or both stemming from a common source) and subsequently yielding the meaning 'lightning-maker'. Another proposal connects Mjǫllnir to Old Norse mjǫll meaning 'new snow' and modern Icelandic mjalli meaning 'the color white', rendering Mjǫllnir as 'shining lightning weapon'. Finally, another proposal connects Old Norse Mjǫllnir to Old Norse mala meaning 'to grind' and Gothic malwjan 'to grind', yielding Mjǫllnir as meaning 'the grinder'.

Likely worn around the neck, the Kvinneby amulet is a small copper amulet found in Öland, Sweden that dates from the 11th century and features an Old Norse Younger futhark inscription that invokes Thor and his hammer. Runologists Mindy MacLeod and Bernard Mees translate the amulet as follows:

The amulet inscription references narratives recorded hundreds of years later in both the Poetic Edda and Prose Edda (see discussion regarding Hymiskviða and Gylfaginning below).

In the Poetic Edda, Mjölnir is mentioned in the eddic poems Vafþrúðnismál, Hymiskviða, Lokasenna, and Þrymskviða. In a stanza from Vafþrúðnismál, the wise jötunn Vafþrúðnir tells the disguised god Odin that after the events of Ragnarök, Móði and Magni, sons of Thor, will wield Mjölnir:

In Hymiskviða, after gaining a tremendous cauldron that the jötunn (and personified ocean) Ægir has requested so that he may brew the gods ale, Thor battles malicious jötnar with the hammer (referred to here as whales as a poetic device):

In Lokasenna, in which the deity Loki and other gods trade insults (see flyting). In the poem, the gods threaten Loki with Mjölnir as part of a refrain repeated in all four stanzas in which he speaks.

The hammer is a focal point of the eddic poem Þrymskviða. In the poem, Thor wakes one day to find that his hammer is missing. Furious, the god pulls his beard, shakes his head, and searches for the absent weapon. Thor consults with Loki, informing him that only he knows that his hammer is missing.