The Modder River basin is part of the Upper Orange River drainage, which encompasses South Africa and Lesotho. The Orange River basin is a cross-border catchment that spans over Lesotho, Namibia, South Africa, and Botswana. The geohydrological mapping of the Modder River Catchment encompasses the identification and classification of rock types, aquifer types, precipitation patterns, river networks, groundwater recharge mechanisms, stratigraphy, and a digital elevation model (DEM). The mapping process utilized a total of 51 monitoring boreholes, both privately owned and government-operated, that were already in place. The western portion of the catchment exhibits efficient drainage because of its interconnected river system, receiving an average annual precipitation of 400–600 mm. In contrast, the eastern sector of the catchment experiences inadequate drainage and receives lower precipitation levels ranging from 300–400 mm. The primary types of aquifers are fractured aquifers and intergranular and fractured aquifers, which have yields ranging from 0.5 to 2.0 L. The annual replenishment of groundwater varies from 0.1 to 12 mm in the western half of the basin, and from 12 to 34 mm in the eastern section. To ensure water security for the residents of major urban areas in the Modder River Catchment, such as Bloemfontein, Thaba Nchu, and Botsabello, it is essential to accurately map and characterize the hydrological and geohydrological features of significant aquifers in the catchment area.