The Mun River Basin is an important agricultural area in northeastern of Thailand and a major tributary of the Mekong River. With the economic and social development, the water environment of the basin has been disturbed, studying the spatiotemporal characteristics of water quality of the Mun River is hence of significance for the protection of the aquatic ecology of the basin and the lower and middle Mekong River. Based on the water quality monitoring data from the Pollution Control Department in Thailand from 1997 to 2017, the seasonal Kendall test method was used to analyze the temporal variation of representative water quality indicators. The water quality index (WQI) method was used to analyze the spatiotemporal characteristics of the water environment. The results indicate that the overall quality of the water environment in the Mun River Basin was good, and no trend has been detected for the water quality in the past two decades. Obvious spatial and temporal variations, however, exists. The water quality of each monitoring site in non-flood season was generally better than that in flood season, and the water quality in the lower reaches of the basin was often better than that in the upper reaches. It was speculated that soil erosion, nutrient leaching and urbanization were important factors affecting the water environment of the basin.