Halda is a 98 km-long tidal freshwater river of Bangladesh, which has a unique feature since it is the only natural breeding ground and the sole source of fertilized eggs of Indian Major Carps globally. The present study firstly summarizes the impact of environmental stressors to understand phytoplankton abundance, community, and their diversity in Halda River, Bangladesh. Environmental data as seasons (Winter, Monsoon), water depths, temperature, pH, Salinity, dissolved oxygen, and nutrients were collected and analyzed from the Halda River in January 2019 and August, 2020.  Thirty-four phytoplankton genera, comprising five classes, were identified from the Halda River, Bangladesh. A two-way ANOVA was performed to analyze the effect of Monsoon and Winter on the physicochemical and biological parameters. It revealed that there was a statistically significant ((F9, 160)= 34.999, p= 0.0) interaction between Monsoon and Winter on the observed parameters. The water temperature (F 125.31, Fcrit 4.130, p= 0.0) and nutrients (F 11.118, Fcrit 2.322, p= 0.0) revealed significant change in the phytoplankton concentrations in the Halda River. As of the temperature and nutrients, water depths also significantly (F 1.790, Fcrit 1.719, p= 0.0) affect the phytoplankton communities in the Halda River showing the highest and lowest cell density in the surface and bottom water, respectively.