Which serpent encircles Midgard according to Norse myth?

Jörmungandr is the serpent that encircles Midgard in Norse mythology.

Also called the Midgard Serpent or World Serpent, Jörmungandr lives in the ocean surrounding Midgard, the human world. According to the Prose Edda, Odin cast the serpent into that sea, where it grew so immense that it could coil around the world and bite its own tail.

Jörmungandr is one of Loki and the jötunn Angrboða’s three children, alongside the wolf Fenrir and the goddess Hel. Odin separated the children because the gods feared the destructive future foretold for them. The serpent’s self-biting coil resembles the ouroboros, although the Norse sources describe their own myth rather than borrowing that later comparative label.

Thor is Jörmungandr’s great enemy. In one story, Thor nearly pulls the serpent from the sea while fishing with the giant Hymir. At Ragnarök, the two finally fight; Thor kills Jörmungandr but dies after taking in the serpent’s venom. Surtr, Fafnir, and Níðhöggr are different mythological figures, not the serpent surrounding Midgard.