Due to the effect of pollution in water bodies such as lakes, lagoons, rivers, among others, the application of methodologies, to assess water quality, lead the analysis of the various components and parameters from altered environmental. In this way, the grey clustering method is a good alternative to evaluate the quality of water, considering that it simplifies a complex system; it groups the same types of factors in their individual categories without losing information. This study evaluated the water quality of the aquatic ecosystem in the Titicaca Lake watershed (Peruvian sector), by the results of the monitoring of water quality in Titicaca Lake, carried out by the National Water Authority (ANA) from Peru. According to the monitoring points, they were divided into four study areas and the parameters studied were chlorophyll content (Chla), total nitrogen (NT), total phosphorus (PT) and chemical oxygen demand (COD). The results of the evaluation showed that 45% of the monitoring points of the Titicaca Lake watershed present an upper mesotrophic level, classified as an acceptable water quality and 55% present a eutrophic level, classified as a poor water quality. This means that the degree of water pollution in the Titicaca Lake watershed is high because there are discharge of sewage and solid waste without any type of treatment. Finally, the results obtained could be of great help to environmental NGOs, local and regional authorities, to improve the environmental management programs in the Titicaca Lake watershed.