The Cornwall Wildlife Trust says it has received hundreds of Portuguese man o' war sightings in the past few days.
It says strong westerly winds are responsible for the influx of the creatures.
Perranporth beach in north Cornwall was temporary closed earlier as a precaution due to the number of specimens found.
CWT says 144 people have reported seeing a man o' war in the past three days.
This compares to only three reported sightings in 2016.
The Portuguese man o' war is often mistaken for a jellyfish but CWT says it is actually a floating colony of tiny organisms living joined together.
The creatures have long tentacles which can cause a painful sting.
According to CWT however stings are only fatal in a few extremely rare cases.
CWT say the number of sightings is "unprecedented" and the public should keep an eye out for any other rare species that wash up.
They also say while people should be cautious around the animals, they should also appreciate their beauty.
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.
Treatment involves removing the tentacles by hand or by spraying with seawater, and immersion in or application of hot water to relieve the symptoms.
Further information on dealing with a man o' war sting can be found on the NHS website.
CWT has provided footage of a Portuguese man o' war swimming around in Cornwall: