This paper discusses the effectiveness of rooftops rainwater harvesting (RRWH) in addressing domestic water scarcity, emphasizing the West Bank (Palestine) as an example of arid to semi-arid areas with limited water resources. The paper deals with the actual and future water demand by considering climate-change impact and urban growth. The analysis is based on the evaluation of (i) the supply–demand balance index (SDBI), which designates the ratio between the total water supply (TWS) and total water demand (TWD), and (ii) the potential of RRWH. Applying this methodology to the West Bank shows that the potential RRWH can contribute by about 40 million cubic meters/year in 2020, which is approximately the same amount of water as the municipal water supply (42 million cubic meters/year). This contribution can effectively reduce the suffering governorates from 64% to 27% in 2020. Furthermore, it can support water-related decision-makers in the arid to semi-arid areas in formulating efficient and sustainable water resources strategies. The analysis also shows that the domestic water scarcity in 2050 will be worse than in 2020 for all governorates. For example, 73% of the West Bank governorates are expected to suffer from extreme to acute water scarcity in 2050 compared to 64% in 2020. Thus, RRWH appears to be highly efficient in mitigating the current and future domestic water scarcity in the West Bank.