The Coastguard is warning people to stay safe after multiple reports of creatures resembling Portuguese Man o War washing up in the West.
Until now, there had been no known sightings in the county.
Volunteers working in Cornwall say they have counted more than three hundred potentially deadly Portuguese man o' war on Portheras beaches.
The jelly-fish like creatures have a powerful sting, which can be fatal.
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It comes after Perranporth beach had to be shut for a short time last week because of the creatures.
The RNLI says people should not touch the creatures.
If you are stung, Their advice is to seek assistance from the RNLI lifeguards on duty.
Cornwall Wildlife Trust says 144 people have reported seeing one of the creatures in the past three days.
There has been a spate of strandings of potentially dangerous Portuguese Men-of-War on beaches in Cornwall and the Isles of Scilly