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The drivers of precipitation variability in the Ethiopian Highlands are complex, and they have considerable impact on the local rural economy of the region. While interannual variability has been studied extensively, relatively less study has focused on intra-seasonal variability in precipitation, even though this variability can be critical for agricultural activities. In this study, we examine the impacts of the Boreal Summer Intra-Seasonal Oscillation (BSISO) on precipitation variability in the Upper Blue Nile Basin (UBNB) during the rainy season (June–September). In order to optimally capture Intra-seasonal variability, we developed a customized ISO index based on leading modes of the variability in the UBNB, and found that this index shares the general character of a standard BSISO index. Analyses performed with both the customized and standard BSISO indices show that a core convective region centered on the Indian Ocean propagates to the northwest, eventually exerting significant influence on Ethiopian rainfall. The primary mechanism of influence appears to be BSISO influence on both lower and upper level tropospheric wind fields over the western Indian Ocean that influence moisture flux, convergence, and convection in the study region. This includes BSISO modulation of the East Africa Low-Level Jet and the Tropical Easterly Jet. We also find that the BSISO influence on Ethiopia is consistent with a BSISO-associated Rossby wave signal, but the magnitude of the Rossby wave signal is small relative to other mechanisms of influence. [ABSTRACT FROM AUTHOR]