Koh Samet is approximately 13.1 square kilometres in size and shaped somewhat like the letter "t". Along the length of the "t" (north-south) the island measures approximately 7 km, and measures 4 km across the "t" (west-east). Forests blanket up to 80 percent of the total area.
Koh Samet gets significantly less rainfall than Rayong Province, even though it is only a few kilometres offshore. but even during this season it has less rain than other islands in Thailand. The island, despite being arid, consists of lush forested hills, covered with evergreen and deciduous forest and cajeput trees grow abundantly.