Whether a rice Green Revolution in sub-Saharan Africa becomes a reality critically hinges on how far productive upland rice cultivation diffuses in the region. In order to quantify the drought tolerance, the rate of water response and the contribution of yield components to changes in yield due to water availability of upland rice varieties used in sub-Saharan Africa, we conducted water application experiments in Namulonge, Uganda, using NERICA 4, NERICA 10, NARIC 2 and Yumenohatamochi, with five different levels of water application. We found that the NERICA varieties were most droughts tolerant, followed by NARIC 2. Yumenohatamochi did not withstand the lowest amount of water application of 378 mm. The results suggested that the minimum water requirement was around 311-400 mm per season for the three varieties used widely in East Africa, and around 420-600 mm for Yumenohatamochi, an upland variety in Japan famous in its drought tolerance. It was estimated that an additional water application of 1 mm increased rice yield by 11-12 kg /ha for the upland varieties tested. The high water response of upland rice was brought about by high water response of four yield components, among which the rate of grain filling contributed most to the increase in yield, followed by number of panicles/m2, number of grains per panicle and 1000- grain weight, in the order of the degree of contribution, for all the varieties tested.