: To estimate the magnitude of quality of Asan River water in Dehradun district, Uttrakhand, India, during post-monsoon season. This study explains the post-monsoon seasonal variation of Asan River water quality with physical and chemical parameters, which identified the potential of pollution sources with similar characteristics. Sixteen Physicochemical parameters such as temperature, turbidity, pH, TDS, EC, BOD, COD, DO, TSS, Total Hardness (Calcium and Magnesium), Sodium, Potassium, nitrate and Alkalinity were performed in post-monsoon season to evaluate the concentration of pollution level in samples that were collected from five different locations of the same river by using experimental and statistical methods. Analytic results showed that all parameters in the water quality index were within the permissible limit parameters like Alkalinity, total hardness; sodium, COD, TDS, and TSS were measured to be higher than the required standards suggested by different agencies such as WHO and ISI during post-monsoon seasons. Analysis enlightens the seasonal changes in Asan River water quality, which always indicates that the pollution is caused by rainfall or other sources of human interference. However, the values of various physicochemical properties vary with seasons. To work on environmental monitoring and waste treatment to protect the population from health risks seems essential.