Fish species composition, abundance, and diversity of Akomoje reservoir drainage basin in Lower River Ogun, Nigeria were studied from June to November, 2017. Water quality parameters were also monitored in-situ and ex-situ using standard methods and kit. One thousand and twelve fish specimen comprising of 14 fish species from 9 families were identified. The Bagrids were the most abundant fish family in the reservoir basin and Chrysichtys nigrodigitatus constituted the most dominant (60.28 %) species and Schilbe mystus was the least abundant species by number (0.59%) and weight (0.74%).. other common species included T. zilli, O. niloticus, C. gariepinus, C. auratus and Malapterurus electricus representing 91.3% while Synodontis budgetti,  Tilapia mariae, Schilbe mystus and S. schall etc. constituted occasional (6.53%) and rare (2.17%) species Diversity indices estimates were Simpson's Index (D) = 0.39, Simpson's Index of diversity (1-D) = 0.61, Simpson's Reciprocal Index (1/D) = 2.56, Shannon-Diversity Index (H) = -1.5749, Shannon's equitability (EH) or Evenness (E) of = -0.5968. Results of physical and chemical parameters measured were air temperature (29.23±1.27), water temperature (26.9±0.37⁰C), dissolved oxygen (6.12±0.70mg/l), and pH (7.6±0.39). There was no significant difference (p> 0.05) in temperature, BOD and phosphorus all though the study period. Negative correlation was determined between water quality parameters and between water quality parameters and fish abundance. The study concluded that the water quality parameters of the study location measured were still within tolerable range for fish survival, however, with the negative correlation obtained between fish abundance and water quality parameters, human activities around this river should be monitored to prevent pollution of the water body. The fish population of this river basin has been well depleted and thus more attention should be paid to the resource to prevent extinction of important fish species.