The Mun River (Thai แม่น้ำมูล) is a tributary of the Mekong river. It carries approximately 21,000 cubic metres of water per year.

It rises in the Khao Yai National Park near Nakhon Ratchasima in the Isan area of Thailand. It then flows east through southern Isan for 418 miles (673 km) until it joins the Mekong at Khong Chiam in Ubon Ratchathani province. The controversial Pak Mun dam is located near the confluence. The Mun river's main tributary is the Chi River, which flows into it in Kanthararom district of Sisaket province.