The Portuguese Man O’War was spotted and snapped by photographer Daniel Blanco as he was riding a stand-up paddleboard near the Costa del Sol town.
A stunned Mr Blanco spotted the venomous creature just a few feet from where a group of children were playing.
Mr Blanco said: “During a sunset ride on a stand-up paddleboard, we found this beautiful but dangerous ocean inhabitant.
“Luckily I had my small underwater camera from me.”
The beach to the east of Marbella Town Centre is also popular with local children who play beach volleyball in the summer.
There were no reports of any beach closures following the sighting.
The creature was off Playa El Cable, where beach club Playa Padre burnt down in February.
Marbella Town Hall sid it would not be offering information on whether there are jellyfish in the sea until the start of the high season on Monday (July 1).
Two of the jellyfish-like creatures were spotted in the water in Benidorm last Sunday.
The first was found at Mal Pas beach, a little cove between the town’s main beaches, and the second was found at the popular Levante beach.
Stings suffered by seven people are said to have been “minor”, although five of the seven sunbathers were taken to hospital. It is not known if any were British.
The Portuguese Man O’War is often called a jellyfish but is actually a species of siphonophore, a group of animals that are closely related to jellyfish.
Their tentacles are loaded with coiled, barbed tubes that deliver venom capable of paralysing and killing small fish and crustaceans.
They are rarely deadly to people but can be dangerous to children, elderly people, asthmatics and people with allergies as they can cause fever, shock and respiratory distress.