The present study examines the diel variation of phytoplankton at Daraganj and Chhatnag, Prayagraj in the Ganga River from December 2020 to April 2021. A total of 57 phytoplankton species were identified including 10 classes (Chlorophyceae, Bacillariophyceae, Cyanophyceae, Trebouxiophyceae, Zygnematophyceae, Coscinodiscophyceae, Cryptophyceae, Euglenophyceae, Dinophyceae and Mediophyceae). Chlorophyceae was the dominant class among phytoplankton followed by Bacillariophyceae and Cyanophyceae. Species richness was highest at 18 o’clock in Daraganj and 6 o’clock at Chhatnag. The high species richness and dominance of Chlorophyceae suggested favourable conditions for green algae, supported by rich nutrient availability. The abundance ranges from 4.0 x105 cells/l to 1.4 x102 cells/l at Daraganj and 2.5 x 105 to 2.3 x102 at Chhatnag. The highest mean phytoplankton abundance was observed at 12 o’clock in both stations suggesting high photosynthesis. Cyclotella sp. was the most dominant species, followed by Scenedesmus ellipticus at both stations. Pearson correlation coefficient of Bacillariophyceae, Chlorophyceae, Cyanophyceae, Dinophyceae, Mediophyceae, Trebouxiophyceae and Zygnematophyceae exhibited a strong positive association with dissolved oxygen (DO) (p < 0.05) and negative relationship with total dissolved solids (TDS) (p < 0.01). Coscinophyceae and Cryptophyceae demonstrated significant negative correlations with TDS (p<0.05). This study underscores the resilience of phytoplankton in response to environmental changes and emphasizes their role as bioindicators for assessing the ecological health of river ecosystems.