Land use land cover (LULC) changes is among the major environmental challenges in the globe. This study aimed to evaluate the impacts of land use/land cover (LULC) changes on the hydrological processes of the Upper Gilgel Abay watershed, Upper Blue Nile Basin. The study used three times series Landsat images, namely Landsat 5 TM 1986, Landsat ETM+ 2003 and Landsat 8 OLI-TIRS 2021. The supervised image classification technique for classifying the Landsat images into LULC classes and the Soil and Water Assessment Tool (SWAT) model for analyzing the hydrological processes were used. The result has shown that an expansion of cultivated land and a reduction of forest, shrubland, grassland and water body were observed during 1986-2003 periods. On the other hand, between 2003 and 2021 periods, cultivated land, grassland and water body were reduced while forest and shrubland were increased. Consequently, surface runoff and water yield were increased during 1986-2003 periods while these hydrological components were reduced in 2003-2021 periods. In contrast, lateral flow, groundwater flow, soil water and evapotranspiration were reduced during 1986-2003 periods and increased during 2003-2021 periods. The findings of this study mainly revealed that the expansion cultivated land on the expense of other land use conversion led to deterioration of the hydrological processes in the study watershed. The result suggests improvements of vegetation cover to reduce surface runoff and increase groundwater in the study watershed in particular and in the Ethiopian Highlands in general.