The objective of this research was to assess the phytoplankton diversity of San Pablo Lake and, based on its composition, determine its current trophic status. Samples were collected from May to August 2023 to account for seasonal variations. Taxonomic identification and photographic records were obtained using an inverted microscope equipped with a camera. A total of 77 species of microalgae and cyanobacteria, belonging to eight phyla, were identified. The most representative groups were Heterokontophyta (31 % of total species), followed by Chlorophyta (25%) and Charophyta (17 %). Cell density peaked in May, reaching 75,535 org/mL, coinciding with the highest rainfall levels. Microcystis aeruginosa, a potentially toxic cyanobacterium, was detected. Nitrogen and total phosphorus levels were elevated compared to other lentic ecosystems in the region. The saprobic index and organic pollution index confirm significant contamination and high organic pollution in San Pablo Lake, classifying it as eutrophic.