Roofs top rainwater harvesting has been carried out worldwide to afford an alternative source of water for many people. The aim of the present study was to evaluate of the rooftop rainwater harvesting potential for alternative supply source in Wachemo University. A 20-years rainfall data was analyzed to determine the average monthly and annual rainfall potential in the study area. The possible volume potential that can be harvested from the selected rooftops of the Wachemo University was quantified using rational method. Ripple mass curve method was also applied to determine the storage capacity of the water tank for the harvestable rooftop rainwater in the study area. Google earth and ArcGIS 10.3 tools were used to digitize the buildings rooftops and to locate water sample points. The quality of the rooftop rainwater was examined as per the standard method for the examination of water and wastewater quality. Considering only the four wet season months (March through October), the monthly variation with coefficient of variation ranges between 33% and 69% and yearly variation between 13% and 24 % .The maximum potential of rainwater with annual average rainfall amount 1188 mm and rooftops area 107587 m2 was found to be 102,268 m3 /y which can be harvested from roofs provided that all rain falling on the roofs is collected. This is equivalent to 52.4 % of average water demand of the community in the university. The Ripple mass curve method showed that about 4269.2m3 of storage tank capacity is essential to store the harvestable rooftop rainwater. The water quality examination revealed that of the qualities of the rooftop rainwater were found within the permissible limit of drinking water quality as per the WHO (2017) guideline except the values of the bacteriological parameters. Total cost for installing the rainwater harvesting system in 94 similar buildings inside the Wachemo University main campus evaluated as 6,937,200 Et.Birr (149,187 USD). The study concluded that rooftop rainwater harvesting is a promising alternative solution in Wachemo University to solve water scarcity problem and for ensuring a continual source of non-potable water.