Koh Seh (Khmer: កោះសេះ, also romanized as Kaoh Seh and its former French name Ile a l'eau) 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. The island constitutes the south-easternmost part of Ream National Park. Koh Seh is sometimes mistaken with Koh Ach Seh, located in Kep Archipelago.