The water quality index is an effective tool for determining water quality. All water treatment plants in the Basrah governorate source their water from the Shatt al-Arab River. A water quality index (WQI) for both raw and treated water for the al-Jubalia water treatment plant is obtained in order to assess its acceptability as a source of residential water supply and the performance of water treatment facilities. From January to December 2019, the physico-chemical parameters were observed for the calculation of WQI for the annual and four seasons: winter, spring, summer, and autumn. The pH, turbidity, electric conductivity, total alkalinity, total hardness, Ca, Mg, Cl, SO4, TDS, Na, and K are the parameters that were considered in this study. In winter, spring, and summer, the results show that raw and treated water were unsuitable for home, industrial, and irrigation needs. In autumn, only the treated water was classified as good water according to WQI categories. As a result, the quality of the Al-Jubalia WTP treated water supply is unfit for human consumption.