Koh Samui Travel Guide Koh Samui, Thailand's third-largest island, is situated 60 km from the mainland in the Gulf of Thailand. Spanning 21 km in length and 25 km in width, Samui was first discovered by backpackers in the 1970s and remains largely covered by lush tropical rainforest and coconut plantations. It is part of the Ang Thong Marine National Park, comprising 80 mostly uninhabited islands....