The bomb, reportedly hidden in a vehicle parked at the Central Samui Department store in Surat Thani province, exploded late last night.
Seven persons were injured in the blast while four others fainted due to shock. Six Thais and a 12-year-old Italian girl were treated for minor injuries and were all released from hospital, according to Poonsak Sophonsasmorong of the island's disaster prevention office.
Police said the vehicle that was parked at the underground car park of the store was completely damaged and had a fake license plate.
It could not be ascertained if the blast was linked to the conflict in Thailand's three southern border provinces where an insurgency has killed over 6,000 people since 2004.
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Around the same time, a fire destroyed the cafeteria building of Surat Thani Cooperative.
The governor said no one was injured in the blaze, that occurred after two explosions were heard.
It was not immediately clear whether the two incidents were linked.
Samui is a popular tourist island in the Gulf of Thailand. Around 20 million visitors visit Thailand each year.