Lake Naivasha is a shallow freshwater lake that supports a wide but uneven biodiversity of plants and animals but no native fish as most of the fish species in the lake were introduced. The lake’s ecosystem has been undergoing major changes due to anthropogenic and natural factors that influence its limnology. This study aimed to determine the diversity, distribution, and biomass of the phytoplankton community in this lake. For a period of 6 months from February 2019 to July 2019 covering dry and wet season, water samples from the lake were analyzed to determine the spatio-temporal trends of phytoplankton and associated environmental variables which included nutrients concentrations, temperature, conductivity, DO, pH and transparency. Monthly samplings were done and samples analyses using the standard procedures. Samples were collected in triplicates for the determination of nitrate nitrogen, soluble reactive phosphorous, total nitrogen, total phosphorous, ammonium nitrogen, silicates, and chlorophyll a. Water temperature, conductivity, DO, and pH were measured in situ using multimeter probe YSI model. The results showed the mean temperature of 22.73 ± 1.6 °C, DO 8.51 ± 0.87 mg/L, conductivity 233.85 ± 26.94 μS/cm, pH 8.13 ± 0.3. TN were 471 ± 170.61 µg/L, PO4-P 5.88 ± 2.12 µg/L, TP 97.97 ± 49.06 µg/L, NO3-N 6.70 ± 3.55 µg/L, NH4-N 18.93 ± 10.91 µg/L and SiO2 3.18 ± 2.99 mg/l and Chlorophyll a 21.51 ± 4.25 mg/m³. Total phosphorous, total nitrogen, silicates, temperature, pH, dissolve oxygen, conductivity, TDS and secchi depth showed significant seasonal differences. A total of one hundred and twenty four (124) species of phytoplankton belonging to six (6) taxonomic families were identified. Chlorophyceae was represented by 43 species consisting of 34.68 % by species composition, Bacillariophyceae was represented by 38 species consisting of 30.65 % by species composition. Cyanophyceae was represented by 24 species leading to 19.35 % species composition. Zygnematophyceae, Euglenophyceae, and Dinophyceae recorded the least species composition. Chlorophyceae recorded the highest total phytoplankton biovolume of 623.41 mm3/L, followed by Bacillariophyceae with 533.16 mm3/L and Cyanophyceae with 114.55 mm3/L. Dinophyceae recorded the least phytoplankton biovolume of 82.36 mm3/L. The total number of algal species was highest at Hippo point sampling site with 72 species, followed closely by 68 species in Crescent Island, then Oserian Bay sampling site with 66 species. Mouth of R. Malewa recorded 59 species, Sher Bay had 58 species, this was followed by Mid Lake station with 56 species, and Sewage Discharge Point had 55 species. Shannon-Wiener diversity (H’) index ranged from 2.0455 at the Mouth of R. Malewa to 2.7077 at Oserian Bay and was highest in July. Lake Naivasha showed a high phytoplankton diversity with significant physico-chemical parameters relationship. There is need for long-term and continuous ecological and hydrological monitoring especially on nutrients seasonal dynamics for proper understanding of seasonal effects of nutrients to algal biomass and diversity, which could be used as an indicator of eutrophication level in the lake.