An eight-year-old was mauled by what experts believed to be barracuda or a shark while swimming off Kamala beach in Phuket on Sunday.
The boy sustained several wounds on his right calf as a result of the attack, according to the Kamala municipality rescue team.
Rescue workers then administered first aid to stabilise the victim for further treatment at Bangkok Hospital Phuket.
Doctors who examined the wound believed the boy had been bitten by a large barracuda, between 80-120 centimetres long.
However, Kongkiat Kittiwattawong, director of Phuket Marine Biological Centre, said the culprit might be a shark that mistook the boy's calf for a fish.
He said bite marks on the leg indicated that the boy might have been bitten by a blacktip reef shark or a bull shark, both of which are found in the Andaman Sea. The sharks are normally spotted in surf zones hunting fish, he added.
Phuket deputy governor Pichet Panapong ordered lifeguards to stay vigilant after the incident. Local authorities have now put up signs along the beach warning people to get out of the water if they spot a big fish swimming nearby.
The boy is now recovering from his injuries, according to Kratu district chief Siwach Rawangkul.
Kamala beach is one of Phuket's most popular beaches. It is located about a 30-minute drive from Phuket International Airport.