Nairobi River Basin (NRB) remains an important ecosystem owing to its endowment with natural resources and its socio-economic value to Kenyans. However, it is vulnerable due to poor resource management, pollution and an increase in the rising population. Environmental flow is thus necessary to assess and plan on how best to maintain adequate quality and quantity of water resources that people in the area depend on. The study used the Tennant and Tessman methods in EF assessment and recommended an annual environmental flow requirement (EFR) of 291 m3/sec equivalent to 37% of mean annual flow. It also recommends an EFR range of 148–577 m3/sec equivalent to 28% and 108% of LF and HF, respectively and a flushing flow of 1,580 m3/sec to be released in October to maintain the river basin's ecological integrity. EF flows in NRB should be corrected using appropriate management measures derived from this study.