|A pattern of water quality affected by the input regime and site specification is one of the most important issues in water supply resources and demand management. The present study aims to assess the water quality of Latyan dam, the important reservoir that supplies drinking water of Tehran, the capital city of Iran. Monthly water sampling was performed at four depths of the dam. The physical and chemical properties of water including temperature, pH, turbidity, total dissolved solids (TDS), electrical conductivity (EC), hardness, as well as Ca2+, Mg2+, Na+, K+, Cl−, SO42−, silica, ammonium, nitrate, nitrite, and total phosphorus (TP) concentration, were monitored from May 2014 to January 2017. The results of statistical analysis indicated that the quality of water in the dam was affected by the depth in view of five variables of EC, pH, turbidity, SO42−, and TP (p ˂ 0.05). Moreover, it was determined that ten out of 17 variables including EC, turbidity, TDS, Ca2+, K, Cl−, SO42−, silica, nitrate, and TP were statistically significant (p ˂ 0.05) based on seasonal variation analyses. Comparing the mean values, it could be concluded that EC, TDS, Ca2+, K+, Cl−, and SO42− were slightly higher during the winter probably owing to the diluting effects of seasonal precipitation. Nutrient-like distribution, nitrate, and TP, as well as silica, were much higher during the spring representing a high-diatoms activity and eutrophication. Turbidity was higher in the fall compared to the other seasons, which also demonstrated rainfall effects. Evaluation of the aggressiveness index (AI) suggests that water is moderately corrosive, and noncorrosive in the winter compared to other seasons. Moreover, according to the findings of this research and their comparison with the national and international water quality guidelines/standards, it could be said that the water quality of Latyan dam during the study period was acceptable and thus it was usable as a drinking water supply. However, due to the changes occurred in nitrate and phosphorus and extensive development of eutrophication problem, accurate and continuous evaluation of water quality in this reservoir is deemed necessary.