Koh Ach Seh (Khmer: កោះអាចម៍សេះ) is a Cambodian island located off the country's southern coast in the Gulf of Thailand.

"Koh Ach Seh" translates to 'Horseback Riding Island', however the name is popular evolving to just Koh Seh - Horse Island.

During the Cambodian–Vietnamese War, the Khmer Rouge used Koh Seh as an outpost.

Military bunkers and fruit trees planted by the Khmer Rouge as a food source are still visible on the island.

[4] MCC has been lent most of the island by the Kep government,[5] except for the very southern part of the island which is home to a small marine police station and fishermen shelters[6][7] Koh Ach Seh is sometimes mistaken with Koh Seh, located inside Ream National Park, Sihanoukville province.