The boy, who was on holiday with his family on the resort island of Koh Phangan, died on Saturday evening.
His death prompted Koh Phangan police to urge hotels and local authorities to warn tourists to watch out for the jellyfish when they swim and to have emergency kits ready in case people are stung.
The box jellyfish, which has tentacles of two to three metres, is considered one of the deadliest creatures on earth.
Koh Phangan is in the Gulf of Thailand near Koh Samui, where the full-moon beach party is attended by hundreds of backpackers.
Local officials say the box jellyfish runs until the end of October.