The Mun River sometimes spelt 'Moon River', is a tributary of the Mekong River. It carries approximately 26 cubic kilometres (6.2 cu mi) of water per year.
The river begins in the Khao Yai National Park area of the Sankamphaeng Range, near Nakhon Ratchasima in northeast Thailand. It flows east through the Khorat Plateau in southern Isan (Nakhon Ratchasima, Buriram, Surin, and Sisaket Provinces) for 750 kilometres (466 mi), until it joins the Mekong at Khong Chiam in Ubon Ratchathani. The Mun River's main tributary is the Chi River, which joins it in the Kanthararom District of Sisaket Province. In Nakhon Ratchasima Province the Mun River flows through Phimai, likely one of the reasons the Khmer sanctuary was built there.