People have been warned not to walk barefoot on Pembrokeshire beaches or touch Portuguese man o' war which have washed up.
Sea Trust Wales said the species of siphonophore - a group of animals related to jellyfish - has a painful sting and in rare cases can be fatal.
The tentacles can be up to 10m (32ft) in length but may be hidden under sand.
They have been seen at Broadhaven and Marloes Sands beaches and could be a sign of warming waters.
"This has been a really epic year for jellyfish," said Sea Trust Wales director Cliff Benson.
"It started in March with millions of small 'by the wind sailors' stranding on our beaches. Later on, during the spring, many thousands of big barrel jellyfish started turning up on our beaches.
"Both were harmless but these Portuguese man o' war are capable of giving extremely painful stings.
"They are usually in the Southern Ocean in the warmer water. We don't usually get them and it could be a sign the water is getting warmer here."
Mr Benson said the first sighting was about 15 miles off Milford Haven a week ago but now they have reached beaches.
In the last day or two, about a dozen were found at Marloes Sands while there have also been sightings at Newgale and Freshwater West beaches.
Mr Benson called on the public to alert them to findings.
"They can sting even when they are dead," he added.
"Don't walk barefoot on the beach, they have tentacles about 10m long and you might see the jellyfish but you may not see the tentacles.
"Certainly you don't want dogs to go near the jellyfish and don't touch them."