| Koh Seh | |
| Native Name: | Central Khmer: កោះសេះ |
| Map: | Cambodia |
| Location: | Cambodia - Southeast Asia |
| Coordinates: | 10.4167°N 151°W |
| Area Km2: | 7.5 |
| Length Km: | 4.4 |
| Width Min Km: | 1.8 |
| Width Max Km: | 4.4 |
| Coastline Km: | 15 |
| Highest Mount: | meters |
| Country: |  Cambodia |
| Country Admin Divisions Title: | Province |
| Country Admin Divisions: | Sihanoukville |
| Country Admin Divisions Title 1: | District |
| Country Admin Divisions 1: | Prey Nob |
| Ethnic Groups: | Khmer |
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 1.50NaN0 south of Koh Thmei and around 90NaN0 south of the main-land of Sihanoukville's Ream commune. A 4.30NaN0 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.[1]
Koh Seh is uninhabited while most of its shores are fringed with mangroves.[2] The island constitutes the south-easternmost part of Ream National Park.[3]
Koh Seh is sometimes mistaken with Koh Ach Seh, located in Kep Archipelago.