Water quality index is used to express the quality of water on both surface as well as subsurface by integrating various water quality parameters. The present work aimed to evaluate seasonal water quality index for Ooty Lake in the Nilgiris district, TamilNadu India. The physico?chemical parameter of water samples taken during two seasons (winter and summer) were analyzed to categorize the level of pollution in the study area. The results were compared with the water quality standards recommended by Environment Protection Act (2009). Based on the Water Quality Index, Multivariate statistical analyses were applied to identify the group of parameter that influence water quality and to evaluate the type of pollution in the lake. In addition to multivariate statistical analyses, the factor and principal component analysis were applied and the outcome showed that there were three types of pollution influencing the water quality namely, agricultural pollution, organic pollution and domestic waste water pollution. During the winter season, domestic waste water pollution was high, as rainwater flushed out in all the areas of watershed. During this season the lake receives a meager amount of water, and hence, the biological activities taking during the summer season has resulted in higher biological pollution. From the analyses, it was observed that there is no major impact by the agricultural pollution. The cluster analysis showed that there were three cluster values, and from the similarity groups among the sampling sites, it was noticed that direct discharge and improper drainage system of watershed caused more pollution in lakes.