Rapid urbanization isknown to have several negative impacts towards hydrological cycle. Urban growth contributes to the increase of impervious area which increases stormwater runoff peak flows and volumes. Nyabugogo catchment has been repeatedly subjected to a growing number of flooding events which resultedin negative effects on water supply system, damage of properties, disruption to business and traffic, discomfort to community, loss of human lives, loss of biodiversity, destruction of environment and deterioration of health conditions owing to water bone diseases. Characterization of stormwaterrunoff is essential in implementing stormwatermanagement and flood mitigation strategies. Hydrological and hydraulic models are used to perform the study and analysis of stormwaterquantity and floods in a catchment. In this study, the United States Environmental Protection Agency’s Storm Water Management Model (EPA SWMM 5.1) was calibrated and validated for stormwater runoff quantity modelling in Nyabugogo catchment. The Geographic Information Systems (ArcGIS) tool was used for catchment delineation; dividing the catchment into sub-catchments and parameterizing the required elements for the model. The collected data from three meteorological stations for period of 1996 to 2017 and three hydrological stations for a period of 1996 to 2017 were used to calibrate and validate EPA SWMM. The performance of EPA SWMM for Nyabugogo catchment was assessed using the coefficient of determination r2, Nash-Sutcliffe efficiency NSE, and index of agreement d. The calibration resulted in r2of 0.72, NSE of 0.6 and d of 0.77 and validation resulted in r2of 0.84, NSE of 0.72 and d of 0.8. The calibration and validation results indicated a good fitbetween simulated and measured data. Overall, the model is acceptable for runoff quantity modelling in Nyabugogo catchment.