Local police told The Phuket News that the woman's body was found floating off Phi Phi Island at about midday on Wednesday local time (6pm NZT).
Tanagon Tammakan of the Phi Phi Island Police said the woman was on Phi Phi Island with a tour group, but refused to return with them.
"She remained on Phi Phi by herself and is believed to have drowned while swimming on Wednesday morning. However, details are still under investigation," he said.
He said the woman was found floating in Lo Dalum Bay by a Myanmar worker on a boat, who notified police.
Her body has been taken to Koh Phi Phi Hospital.
Tanagon says a hospital examination revealed no signs of violence and the doctor ruled the cause of death as drowning.
It was believed the woman had been dead for about six hours before she was found.
"She was identified because the tour group contacted police after receiving news reports that the body of a foreign woman had been found. They then confirmed her identity through her tattoos," said Lt Tanagon.
The woman's family have been informed he said.
Thai police are not releasing the woman's name.
