Jellyfish are marine creatures existing on Planet Earth and in Monster World. Scyphozoa is the class most often atributed to jellyfish in fiction and is instantly the most recognisable with his cup-shaped body and long, stinging tentacles trailing underneath it. They are eaten as a delicacy in China.

Jellyfish are fairly rare in Sonic the Comic:

• Shion uses a jellyfish as a stepping stone when it inexplicably leaves the water to attack him in Wonder Boy in Demon World. Its dome-shaped body is ideal as a springboard.

• The first Ecco the Dolphin story sees Ecco engage in battle with a jellyfish in frozen waters. The "fingerfish" ensnares the dolphin, but Ecco soon gets away, only to be confronted with a polar bear. Using his strong fins to break the ice, Ecco pits the two predators against each other, the jellyfish attempting to poison the bear while the "walker of the water shell" goes in with the teeth. The jellyfish is suggested to be the loser in the battle, possibly dying.