Koh Tonsay (Khmer: កោះទន្សាយ) is located about 4.5 Kilometers southwest of Kep. Tourists are drawn to the two beautiful white sand beaches, and the sea is shallow excellent for swimming. At the sea bottom are a variety of corals, sea animals and plants which attract researchers and ecologists.

The name Koh Tonsay is derived from the word Rumsay. While trying to avoid the commander’s troops, Prince Sakor Reach grew hopeless because his own troops began to tire. He led his remaining troops across the sea to an island in front of Kep city, where the troops spread out. Accordingly, the island was called Koh Rumsay, Koh Ormsay, Koh Ornsay, and Koh Tonsay, as it is known today. Another version is that, according to many locals who spent there youth in Kep before 1975, the island was previously called Koh Antai. There is no apparent meaning for this word.

Koh Tonsay is 2 square kilometers in area. During then-Prince Norodom Sihanouk’s Sangku Reastr Niyum regime, it was used as a place to rehabilitate criminals, who were also used to defend the island. Horse cart paths and wooden, thatch roofed motels were also constructed during this time. Most of this infrastructure has been destroyed by weather and decades of war. Today, seven families live on the island. They earn their living by fishing and growing coconut trees.