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.
- 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.
- Blue, Pink and Green Jellyfish：“That strange tingling sensation in the water may be the unwelcome surging of electrical death from a brainless jellyfish. Swim with care.”
- 血水母：“Jellyfish swimming in the bloody waters of the Crimson become infected, converting them to its all-consuming cause.”
- 蘑菇鱼：“Some fish exposed to glowing mushroom spores become a curious mushroom-jellyfish hybrid, glowing softly in the watery depths.”
- 电脑版 126.96.36.199：Blue Jellyfish now has a chance to appear in Hardmode.
- 电脑版 188.8.131.52：Fungo Fish sprite updated.
- 电脑版 1.3.5：蓝水母、粉水母、和绿水母的外观更新。
- 电脑版 184.108.40.206：修正了专家模式中在你无法击中水母时水母计为击中你的问题。
- 电脑版 220.127.116.11：
- Shock attack introduced.
- Blue Jellyfish damage increased from 20 to 25, and life from 30 to 34.
- Green and Pink Jellyfish now drop their own respective banners.
- 电脑版 1.1：引入绿水母。
- 电脑版 1.0.5：引入。