<p>Green buildings, as highly efficient and energy-saving structures, aim to reduce reliance on conventional energy sources, lower carbon emissions, and enhance energy utilization by harnessing renewable resources like solar and wind energy. Photovoltaic-thermal systems and rainwater harvesting systems are pivotal sustainable infrastructures in green buildings. However, two critical factors impede the further enhancement of building energy efficiency. First, the overheating of photovoltaic-thermal (PVT) components reduces their conversion efficiency. Second, rainwater systems face water supply–demand mismatches. Furthermore, both systems compete for the limited space available on green roofs. The rainwater photovoltaic-thermal systems represent an emerging technology that integrates photovoltaic solar thermal systems with rainwater harvesting systems on building rooftops. This integration effectively addresses the overheating issue associated with PVT modules by designing rainwater pipes to circulate through the modules for cooling purposes. Additionally, the electricity generated by the photovoltaic panels powers the energy-intensive components of the rainwater systems. Meanwhile, the specific inclination angle of the photovoltaic panels enhances rainwater harvesting efficiency, thereby facilitating mutual resource utilization. This paper critically reviews the integrated rainwater photovoltaic-thermal systems to determine whether installing the coupled system on building rooftops is more energy-efficient than installing the two systems separately and whether it better supports the sustainable development of green buildings. By analyzing the independent opportunities and challenges of each system, this study concludes that the coupled system significantly contributes to improving building energy efficiency, reducing consumption, and advancing green building development.</p>