| Tonlé San | |
| Name Other: | Ya Krong Po Ko |
| Source1 Coordinates: | 14.6063°N 107.8404°W |
| Mouth Coordinates: | 13.536°N 105.965°W |
| Subdivision Type1: | Country |
| Basin Size: | 17000km2 |
Tonlé San (Central Khmer: ទន្លេសាន,), also known as the Sesan River (Vietnamese: Sông Sê San), is a river that flows through central Vietnam and north-east Cambodia. It is a major tributary of the Mekong River. Its tributaries include the Dak Bla, Dakpsy, Sa Thầy and Lagrai rivers.[1] A short portion of the river forms a part of the international border between Cambodia and Vietnam.
There are a number of hydropower dams on the Se San River and its tributaries. Where it joins the Srepok River in the Lower Se San 2 Dam. Upstream is the dam cascade: Se San 4A, Se San 4, Se San 3A, Se San 3, Yali Falls. On the Dak Po Ko River is the Plei Krông dam. There are also several dams on tributaries of the Dak Bla, including the Dak Snghé, tributary of the Dak Bla, is the Upper Kontum dam and hydropower plant, which discharges into the Tra Khuc River, and the Dak Doa.[2]