This study is aimed to investigate the water quality condition in Kelani River downstream. The secondary data collected from the ‘Pavithra Ganga Program’ was used to evaluate the existing water quality condition in Kelani River and QUAL2k modelling software was used to predict the water quality within the selected upstream and the downstream. The model was calibrated and validated by inputting kinetic parameters, point source data, non-point source data and geometrical data. The main concern was given to water quality parameters like dissolved oxygen (DO), biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total Nitrate (TN) and total phosphate (TP).After the validation, calibrated model was used to predict the water quality along the Kelani River. It was revealed that BOD5/COD ratio increased from upstream to downstream indicating that the biodegradable waste is more prominent in downstream. Even though the quality of water at Ambatale intake is currently within the proposed inland water quality standards for drinking and conventional treatment, the situation may rapidly vary due to emission of pollutants by non-point sources resulting in the increased population along the river and expansion of the residential area. The calibrated QUAL2k model for Kelani River can be used for planning and managing the Kelani River downstream.