Rainwater harvesting (RWH) systems are effective in alleviating water supply shortages, while green roofs (GRs) can contribute to stormwater management, air quality improvement, thermal regulation of buildings, and biodiversity support. Despite their individual benefits, both systems are not frequently combined. This paper investigates the potential for integrating these systems through a hydrologic modeling and optimization approach, using a case study in Paris, France. The study utilized a Conceptual Interflow model (CI-model) coupled with a Water Balance (WB) model to describe the rainfall-runoff relationship of integrated green roof and rainwater harvesting (GR-RWH) systems. An NSGA-II optimization was then applied to the CI-WB model to determine the optimal tank sizing of GR-RWH systems for meeting different water demands. The results show that GR-RWH systems have water reliability (WR) values similar to those of traditional RWH systems without GR, albeit with larger tank volumes. For new buildings in Paris, a GR-RWH system with approximately 25 to 75% GR coverage meets rainwater utilization needs with low investment while also providing the added benefits of GRs.