Jörmungandr (Old Norse: Jörmungandr, meaning ‘Mighty Serpent’) is the World Serpent, son of Loki and the giantess Angrboða, who encircles all of Miðgarðr with its body and bites its own tail. Odin cast the serpent into the ocean surrounding Miðgarðr, where it grew to such a size that it can encircle all the land. It is the fated adversary of Thor.

Jörmungandr and Thor’s enmity is the subject of multiple encounters. In Gylfaginning, Thor nearly catches the serpent on a fishing line while visiting the giant Hymir — the same episode described in Hymiskviða — but Hymir cuts the line before Thor can land it. The serpent is also encountered in Gylfaginning’s account of Thor’s journey to Útgard, where Útgarða-Loki disguises the World Serpent as a giant cat that Thor fails to fully lift. At Ragnarök, Thor kills Jörmungandr but then walks nine steps and dies from its venom — consistent across Völuspá and Gylfaginning.

The Thor-Jörmungandr fishing scene is among the most widely depicted mythological episodes in Viking Age material culture, attested on the Hørdum stone, the Altuna runestone, and other carved monuments.