Koh Samui Island, Thailand. Koh Samui, or often simply Samui, as it is referred to 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. Ko Samui is Thailand's second-largest island after Phuket. Abundant natural resources, white sandy beaches, coral reefs and coconut trees are present on the island.