Locals were alarmed when blue jellyfish appeared in swarms near the shores of Thailand.

Footage shows one of the vividly coloured invertebrates drifting along the coast of Phetchaburi province on August 26.

Jellyfish sightings have terrified residents in recent weeks after a Thai tourist was stung by a Portuguese Man o'War while swimming off Koh Lanta island in southern Krai province in late July.

The man, Prakan Boonsri, was with his family strolling along Phra Ae Beach when the tide swept the siphonophore toward his feet. Its sting had left him with painful rashes where the creature's tentacles came into contact.

Following the incident, Neramit Songsaeng, chief of the Koh Lanta National Park, said he had received reports of Portuguese Man o'War sightings from hotels in the Koh Lanta district the week prior.

Locals had been warned to be vigilant when entering the water.

A Portuguese Man o'War, though appearing to be a jellyfish, is a colony of smaller creatures called zooids. They function as a single organism and are found on the surface of the warm tropical seas. Its distinctive feature is a gas-filled 'balloon', with long tentacles - capable of delivering painful stings - extending underwater.