A German woman has died after being stung by a box jellyfish on a popular Thai resort island, the third reported fatality in Thailand in 14 months from one of the world's most toxic jellyfish.

The 20-year-old woman was taking an evening swim at a Koh Samui beach when she was stung by the jellyfish, whose sting can kill an adult within minutes, police Lt Col Thewet Pruemsut said.

A friend of the woman, also a German tourist, was stung on the hand by a jellyfish but survived.

The other two recent deaths from box jellyfish occurred on another popular island in the Gulf of Thailand, Koh Phangan, known for the notorious Full Moon Party. A Thai woman died there after being stung in August, while a five-year-old French boy died from a sting in August 2014.