The blast occurred about 11.30pm at the Central Festival shopping mall.
Local television reports showed damaged cars and injured shoppers.
Governor Chatpong Chatput said the explosion was caused by a car bomb placed inside a pickup.
Initially, Pol Col Apichart Boonsriroj, police commander of Surat Thani province on the mainland, said this morning, “It is not clear whether it was a car bomb or a gas explosion. The bomb squad will investigate this morning.
“Seven people were slightly injured by shrapnel,” he said, adding that all had been released from hospital.
However, AFP quoted Lt Gen Prawut Thavornsiri, spokesman for Thai national police, assaying later, "It's a car bomb but we cannot confirm what type of explosive materials they used."
A Samui official said six of the injured were Thai and the seventh was a 12-year-old Italian girl.
“Six Thais and a 12-year-old girl were treated for minor injuries,” said Poonsak Sophonsasmorong of the island’s disaster prevention office.
See original story here.