Pool of warm water and flower petals at a spa in Ko Samui, Thailand. Photo by Natala Menezes. Ko Samui, often referred to as simply Samui by locals, is an island off the east coast of the Kra Isthmus in Thailand. It is located close to the mainland town of Surat Thani in Surat Thani Province. It is Thailand's third largest island after Phuket and Ko Chang. Ko Samui has abundant natural resources- white sandy beaches, coral reefs and coconut trees.