The river system is the most complex of the freshwater bodies in the world and is prone to pollution especially from anthropogenic activities and the quality of water, as partially determined by physicochemical properties, is very crucial for primary productivity in the aquatic environment. This study determined the physicochemical characteristics and green microalgae of water collected from Majopa, Ogunpa and Uren rivers in Ogun State, Nigeria. Surface water samples were collected from three different rivers using direct collection method into the labelled bottles for physicochemical characteristics determination, culturing and ecological studies. Water samples for ecological studies were fixed in 4% unbuffered formalin while sample for culturing were not fixed but growth stimulated using Bold Basal Medium (BBM) and Blue- Green Medium (BG-11). The physicochemical parameters varied widely with the profile of water sample from Ogunpa River being the most favourable for widest diversity of green microalgae of all three. The pH of the water samples were slightly acidic ranging between 6.3 and 6.5 for the rivers, while the lower dissolved oxygen (2.61-3.01 mg/L) recorded pointed to a stressed environment which probably accounted for lower number of microalgae taxa observed, especially in Majopa River. The ecological studies showed the presence of ten taxa from the division Chlorophyta with nine taxa from Ogunpa River and Scenedesmus sp. being dominant genus across the rivers. The microalgae diversity of the river water was Ogunpa River˃Uren River˃Majopa River. BBM supported growth of the microalgae much better. The physicochemical properties portrayed Majopa and Ogunpa River water as unfit for drinking, while that of Uren River is relatively fit for drinking. It is highly recommended that the microalgae reported therein be further characterised for various possible economic benefits. Keywords: Growth, Microalgae, Nutrient, Ogun State, River, WaterDepth, Geothermal Energy