"It covers about 300 meters (990 feet) of the beach. That's a lot," Soomet Saitong, chief of the Khao Laem Ya National Park which includes Samet island, told AFP.
Some visitors were cutting short their holidays on the island, which is a popular destination for weekend breaks for Bangkok residents.
"There are oil stains right in front of the beach. Customers are starting to check out," a worker at the Ao Phrao Resort told AFP.
In this photo, workers prepare to pump oil out of the surf on Ao Prao island on Koh Samet island.