Ko Samet^[1] (Cajeput Island; Thai: เกาะเสม็ด, Thai pronunciation: [kɔ̀ʔ samèt]) is one of the eastern seaboard islands of Thailand. It is in the Gulf of Thailand off the coastline of the Thai province of Rayong, approximately 220 kilometres (140 mi) southeast of Bangkok. Ko Samet is part of the Phe (Thai: เพ) Subdistrict of the Amphoe Mueang Rayong (Thai: เมืองระยอง, the capital district, or Amphoe Mueang), of Rayong Province. Ko Samet is the largest and westernmost of a cluster of islands not far from the coast. Ko Samet measures 6.8 kilometres (4.2 mi) from north to south. At its closest point to the mainland, the island is 2.6 kilometres (1.6 mi) south of mainland Rayong Province; however the actual travelling distance from the main commercial tourist pier in the town of Phe to the island’s closest tourist beach is about 10 kilometres (6.2 mi).

Popular with foreign tourists and as a weekend getaway for residents of nearby Bangkok, most of the island (excluding the Na Dan area) is in the Khao Laem Ya–Mu Ko Samet National Park. In 1981, the Royal Forest Department declared the archipelago of Samet, along with nine other small islands, the headland of Khao Laem Ya, and the 11 kilometres (6.8 mi) Mae Ramphueng Beach to be a national park, and thus Ko Samet is under the protection of the National Parks Division of the Thai government.