<p>Water eutrophication is a significant environmental issue that impacts aquatic ecosystems worldwide. In this study, we aimed to investigate the relationships among water physicochemical characteristics, the phytoplankton community, and cyanotoxins in the Ubol Ratana Reservoir, Khon Kaen, Thailand, from 2022 to 2023 and to evaluate the relationships between changes in water parameters and the dynamics of the phytoplankton community under seasonal variations. Eighty phytoplankton species belonging to eight phyla were found in six different locations in the reservoir. Cyanobacteria had the highest cell abundance (84.05%), with <i>Microcystis aeruginosa</i> being the dominant species. With respect to the seasonality of the diversity indices, the Shannon‒Wiener and evenness values were highest during the summer and lowest during the rainy season, whereas the species richness remained constant. Interestingly, the phytoplankton abundance was significantly greater at the center of the reservoir and the water outlet than at the other sampling locations, and these two areas presented the lowest nitrate and ammonium concentrations. Canonical correspondence analysis (CCA) indicated that nitrate, ammonium, and water inflow are factors that influence phytoplankton growth. Correlation analysis revealed that the abundance of dominant cyanobacteria was positively correlated with the total phosphorus and orthophosphate concentrations and negatively correlated with the nitrate and ammonium concentrations. Overall, the nitrogen concentration was the dominant factor regulating cyanobacterial blooms. These results suggest that relationships between physicochemical factors and the phytoplankton community significantly influence the seasonal variations in phytoplankton communities, providing valuable insights for future monitoring of phytoplankton blooms.</p>