The bomb, reportedly hidden in a vehicle parked at the Central Samui Department store in Surat Thani province, exploded late last night.
Five persons were injured in the blast while four others fainted due to shock. An Italian girl was among the injured.
Police said the vehicle that was parked at the underground car park of the store was completely damaged and had a fake license plate.
Surat Thani Governor Chatpong Chatphum said the blast also damaged over 10 other cars.
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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.