Seasonal changes in water quality and phytoplankton composition in Bung Binh Thien Lake, An Giang province were investigated in this study. Nine samples were collected in the dry and rainy season in 2019 for analyzing the composition of phytoplankton and identifying the principal water quality factors influencing the phytoplankton. Temperature, turbidity, pH, total suspended solid (TSS), dissolved oxygen (DO), biological oxygen demand (BOD), chemical oxygen demand (COD), ammonium (N-NH4+), orthophosphate (P-PO43-) and coliforms were analyzed for water quality index (WQI) determination. The results indicated that five phyla of phytoplankton including Bacillariophyta, Chlorophyta, Cyanophyta, Euglenophyta and Dianophyta were identified at the lake. Species compositions of chlorophyta was the most abundant while Dinophyta were found in limited numbers in the two seasons. However, Bacillariophyta was the highest density thus dominating in the water through seasons. Season resulted in strong influence on the phytoplankton, for example, density and Shannon-Wiener diversity index (H’) in the dry season were higher than those in the wet season. Both H’ and WQI indexes revealed that water quality in the dry season was less polluted than that in the rainy season. Canonical Correspondence Analysis (CCA) showed that DO, turbidity, TSS and nutrients (P-PO43- and N-NO3-) were the main factors affecting phytoplankton composition in the lake. This study provided important information on interrelation of phytoplankton composition and water quality parameters supporting water monitoring.