Factors influencing the distribution and variations of abundance of phytoplankton of dams are yet to be accounted for in Southwest Nigeria. Therefore, the objective of this paper was to evaluate the seasonal distribution and variations of abundance of phytoplankton community in Ikere Gorge Dam, Oyo State, Southwest Nigeria using standard techniques. Data obtained show that a total of 494 microalgal taxa belonging to eight divisions were recorded. Zygnematophyceae comprised 360 taxa (64%), Bacillarophyceae 22 taxa (14%), Chlorophyceae 66 taxa (13%), Cyanophyceae 20 taxa (4%), Dinophyceae 8 taxa (1%), Trebouxiophyceae 13 taxa (2%), Ulvophyceae 3 taxa (0.02%), Xanthophyceae 2 taxa (0.01%). The observations and results confirm hypotheses that phytoplankton abundance varies more between season (wet and dry) than spatially in Ikere gorge dam. No single taxon or a combination of two or three taxa accounted for more than 80% of the phytoplankton abundance. Canonical correspondence analysis revealed DO, TDS, nitrate-nitrogen and water transparency as the major water quality variables driving variation in the composition of plankton communities in the dam. This study showed seasonality is the major factor influencing the diversity and abundance of phytoplankton community through changes in concentrations of DO, nitrates, and phosphate. Ikere Gorge dam showed a strong seasonal variation in physico-chemical water quality variables owing to the size of the dam that is not well-mixed, and with long water residence times. This study contributes to understanding the water quality, determinant factors, and drivers of biological communities in dams of tropical regions that are being influenced by anthropogenic activities.