What sea serpent does Thor fight at Ragnarok?

The sea serpent that Thor fights at Ragnarök is Jörmungandr, also called the Midgard Serpent or World Serpent.

Jörmungandr is the middle child of Loki and the jötunn Angrboða. According to the Prose Edda, Odin throws the serpent into the ocean surrounding Midgard. It grows so enormous that it encircles the world and bites its own tail, an image associated with the ouroboros.

Thor and Jörmungandr are longstanding enemies. In one famous story, Thor goes fishing with the jötunn Hymir, uses an ox head as bait, and hooks the serpent. Thor prepares to strike it with his hammer, but Hymir cuts the line and the monster escapes. The episode is preserved in several Norse poems and became a popular subject in Viking Age art.

At Ragnarök, Jörmungandr releases its tail, floods the land, and confronts Thor. Thor kills the serpent but walks only nine steps before dying from its venom. Fenrir, another child of Loki, is the creature associated with Odin’s death.