The jellyfish-like creatures (which are not actually jellyfish) with purple tentacles have washed up on the beaches in the north of the Finistère department in recent days.
The sea creatures can cause burns, fever, cramps, loss of consciousness, or even heart failure. Local media in Brittany have reported that authorities have banned swimming on certain beaches until they can give the all clear.
This bizarre looking marine animals are usually found in warmer waters, but it is thought they drifted into Brittany's waters in strong currents.
So far, there have been sightings at the Saint-Laurent beach, the Mazou port at Porspoder, the Porsmeur beach at Plouescat and in the Baie des Trépassés on Cap Sizun.
This is not France's first invasion of the Portuguese Man-of-War - the marine creatures washed up on shores further south in the Gironde department on the western coast in 2011.
One Cornish beach in the UK also had to be closed this year due to these Portuguese Man o' War appearing on the sand, which experts have called the biggest stranding of the deadly creatures there since 2012.
Anyone who comes into contact with these venomous creatures is advised to seek urgent medical help.
By Anna Schaverien