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 (1 mi) south of Koh Thmei and around 9 km (6 mi) south of the main-land of Sihanoukville's Ream commune.

A 4.3 km (3 mi) wide sound separates it from Phu Quoc.

"Koh Seh" means Horse Island.

[1] The island constitutes the south-easternmost part of Ream National Park.