Photographs taken by Tiger Cox shows the large animal floating in the sea in Seaford.
Sussex Wildlife Trust confirmed it is likely to be a barrel jellyfish.
The UK’s largest type of jellyfish, they often swarm in warmer coastal waters in late spring.
They often wash up on beaches across the country in May or June, sometimes in their hundreds.
A barrel jellyfish (Image: Tate Grasso/Wikimedia Commons)
Their favoured food is plankton, which can be found in shallow waters and is often the reason for them washing up on beaches.
Sussex Wildlife Trust said the sea creatures have eight “frilly” arms, which contain their small stinging tentacles and surround “hundreds of little mouths”.
Although their sting is not normally harmful to humans, it is advised not to handle them as they can still sting when they are dead.
Barrel jellyfish are also the favourite food of leatherback turtles.
Sussex waters are home to a range of interesting animals, with the public often sharing their sightings.
A woman believed she had spotted a shark fin near Worthing Pier (Image: Rebecca Hoibak)
Rebecca Hoibak was strolling along Worthing Pier when she spotted a strange shape in the sea.
At first, she believed it was a shark’s fin but later discovered it was a seal.
In March, Brighton and Hove Seafront Team rescued a catfish which was stranded on Brighton beach.
Team member Chaz helped to rescue the animal, highlighting that the sighting was “really unusual” due to the low tide at the time.