Jörmungandr (Old Norse: Jǫrmungandr - "huge monster"), also known as the Midgard Serpent (Old Norse: Miðgarðsormr), is a sea serpent, the middle child of Loki and the giantess Angrboða. According to the Prose Edda, Odin took Loki's three children by Angrboða (the wolf Fenrir, Hel, and Jörmungandr) and tossed Jörmungandr into the great ocean that encircles Midgard. The serpent grew so large that it was able to surround the Earth and grasp its own tail, receiving the name of the Midgard Serpent as a result. Upon the onset of Ragnarök, Jörmungandr will release its tail, as he is met in battle by his arch-enemy, Thor, god of thunder.

Thor and Jörmungandr from the Altuna runestone at Altuna church, Enköpings municipality, Sweden.