Koh Seh is a Cambodian island located in the Gulf of Thailand, inside Ream National Park, Sihanoukville province. It lies 1.5 km south of Koh Thmei and around 9 km south of the main-land of Sihanoukville's Ream commune. A 4.3 km wide sound separates it from Phu Quoc. "Koh Seh" means Horse Island. Less than 400 meters south-west of Koh Seh lies the tiny islet of Koh Ky. Koh Seh is uninhabited while most of its shores are fringed with mangroves.