Lizzie Daly came across the huge barrel jellyfish while diving near Falmouth on Saturday.
She said the sea creature was as big as her body and the largest of its kind that she had seen.
The biologist and wildlife presenter was diving as part of her Wild Ocean Week campaign, which aims to celebrate our oceans while raising money for the Marine Conservation Society.
Barrel jellyfish are the largest species of jellyfish found in UK waters.
They normally gravitated towards warmer coastal warmers in late spring and often wash up on British beaches in May or June, sometimes in their hundreds, according to The Wildlife Trusts.
They get their nickname of “dustbin-lid jellyfish” from the fact they can grow to the size of a dustbin lid.
Their sting is not usually harmful to humans, but they can still sting when dead.
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