"Despite only being a couple of hours south of Phuket and even closer to Koh Phi, Koh Kradan is a world apart, sharing all the natural beauty but without the crowds and noise," World Beach Guide explains.
Koh Kradan is one of about 40 islands and islets forming a major part of the Hat Chao Mai National Park.
"Of all these islands and islets, Koh Kradan is perhaps the most beautiful and largely unspoilt," World Beach Guide said.
The translucent sea around the island is perfect for snorkeling and kayaking, while its beaches are a sunbather’s delight, it said, noting as well that visitors have unsurpassed views of the sun setting over the Andaman Sea.
Koh Kradan has several small resorts, ranging from the upmarket but basic “flash-packer” style to luxury bungalows.
It was followed on the list by French Polynesia's Matira Beach, Australia's Whitehaven Beach, the Philippines' El Nido, and the British Virgin Islands' The Baths.
Four more Thai beaches made the top 100: Krabi's Railay Beach (9th), Phuket's Freedom Beach (18th), Phang Nga's Laem Had Beach (21st), and Surat Thani's Ao Tanote Bay (44th).