Morgan said he was swimming near Trefin Beach in Pembrokeshire when he spotted the tentacles in the water.
“Three turned to 10, and then to 20. They just kept on coming,” he added.
The jellyfish was so big the tentacles went “beyond the range of visibility”.
He initially thought it was a compass jellyfish, which are native to UK waters, before realising it was too big.
“It must have been the length of a bus or small train. I couldn’t believe how big it was.”
The Wildlife Trusts, external said the lion’s mane jellyfish gets its name from its long tentacles, and have “all the beauty and bite of a lion”.
They can grow up to 3m (9ft 8in) in length, but the tentacles – which are used to catch their prey – can exceed 30 to 36.5 metres (98ft 4in to 119ft 8in) making it one of the longest animals on Earth.
Marine biologist Dr Cathy Lucas, from the University of Southampton, said Morgan’s video showed the animal’s incredible length.
“The tentacles are so long and so fine you might not even see them,” she said, recalling how during field work in Scotland divers would get bits of tentacles on them when there were no jellyfish around.
“They still sting after they break off,” she said, adding that while the sting was “nasty” and “the worst in the UK”, it was not dangerous.
