This article is about the enemies. For the collectible bait items, see Jellyfish (bait).
Jellyfish are enemies found in water. When underwater, they attack by propelling themselves towards the player in short bursts. When outside of water, they will be unable to move or pursue the player in any way. Jellyfish are able to jump out of the water to attack players, often leaving them stranded on land. Most varieties of Jellyfish produce light, making them easy to spot in dark areas. There are currently 5 Jellyfish variants, 3 of which only appear during Hardmode. Blue Jellyfish only spawn in pre-Hardmode and are replaced by the Green variant in Hardmode.
In Expert Mode, Jellyfish will periodically become electrified, making them invincible for the duration of the attack. If the player attacks a Jellyfish with a sword while it is electrified (or any other swung item that deals direct damage, such as most tools), the player will be hurt. A Jellyfish will constantly maintain its speed while electrified unless it comes into contact with a wall or the water's surface.
All Jellyfishes are immune to the Confused debuff.
If knocked out of water and onto dry blocks, Jellyfish are rendered immobile until knocked back into water, or water reaches them again. In expert mode, this will also prevent them from becoming electrified.
Blue and Green Jellyfish are capable of spawning in the Underworld if there is a large enough body of water.
In Expert Mode, dying from electrocution damage via direct melee contact will not attribute the cause of death to the Jellyfish in the resulting death message. It will be a default message with no cause listed.