The supply-demand gap of potable water in rapidly growing urban cities in developing countries has become a major challenge. This is potentially worsened by poor and superficial assessment of the complex parameters of water resources and treatment technologies regarding capacity expansion. The estimated daily demand of 150 million gallons in the Accra-Tema Metropolitan Area (ATMA) region of Ghana far outweighs the daily supply of 94 million gallons. In this study, the Analytic Hierarchy Process (AHP) approach, through the inputs of experts in the water industry in Ghana, is used to assign weights to the identified significant factors that impact on selecting the best alternative source for urban water supply capacity expansion in the ATMA region. Three alternative plants, alongside their respective sources, are considered — the Weija, the Kpong, and the Teshie Desalination plants. The decision criteria considered are environmental, economic, technical, and socio-cultural criteria, with each having sub-criteria. In analysing the pairwise comparative judgments by the experts, the environmental criterion was found to be the most important criterion with the highest priority weight, followed by the economic, the technical and the socio-cultural criteria. In the analysis, the Kpong treatment plant ranked first with a score of 36.1%. This was followed by the Weija and Teshie desalination plants, which scored 33.8 and 30.2% respectively. Sensitivity analysis on the model revealed that the model is sensitive to the environmental and economic criteria while being robust to the technical and socio-cultural criteria. Sensitivity, in relative terms, indicated that the resource availability sub-criterion is the most critical, while that of the energy sub-criterion proved the most critical in absolute terms.