The growing demand for reliable, low-carbon energy in the Southern African Power Pool (SAPP) necessitates a harmonized, region-wide approach to hydropower development in the SAPP. While GIS–MCDA and AHP methods have been widely applied at basin or national scales, this study uniquely presents a comprehensive geospatial analysis by integrating multiple spatial datasets across all 12 SAPP member states: Angola, Botswana, Democratic Republic of the Congo (DRC), Eswatini, Lesotho, Malawi, Mozambique, Namibia, South Africa, Tanzania, Zambia, and Zimbabwe. This study advances existing research by applying a standardized GIS–MCDA–AHP framework consistently across diverse countries, incorporating underutilized criteria such as the Standardized Precipitation Index (SPI) and detailed geological and soil texture layers, and integrating environmental and land-use constraints through Boolean masking. The resulting Hydropower Suitability Index identifies spatially coherent, high-potential hydropower corridors along major river systems, including the Congo, Zambezi, Cuanza, and Limpopo basins. Validation with existing hydropower stations and a sensitivity analysis confirmed the robustness of the model. The findings provide policy-relevant insights that support cross-border energy planning, sustainable regional energy expansion, and enhanced climate resilience in Southern Africa, offering practical guidance to policymakers and planners.