Green roofs play a vital role in urban climate adaptation by reducing heat stress and stormwater runoff. However, their effectiveness is highly dependent on substrate moisture. During dry periods, extensive green roofs rapidly dry out, significantly diminishing their cooling capacity. This restricts plant selection to drought-tolerant species such as succulents, which offer only limited evaporative cooling potential and biodiversity benefits. Supplemental irrigation can enhance green roof cooling performance and may allow the cultivation of more diverse and transpiring vegetation. However, excessive or poorly timed irrigation may reduce the system’s capacity to retain stormwater. In this study, a novel smart irrigation system was developed, which is based on the assimilation of weather forecast data into a hydrological model to allow for demand-driven water supply. The aim of the smart irrigation management is to avoid water stress for plants and provide cooling only on warm days, while targeting to maximize the retention capacity before rainfall events to achieve the effective interaction of multiple ecosystem services. The performance of the smart irrigation system is compared to conventional irrigation approaches relying on fixed intervals or simple soil-moisture threshold controls. Irrigation substantially enhanced daytime surface cooling, reducing surface temperatures by up to 6.85 K compared to the non-irrigated roof. While irrigated green roofs offered thermal regulation on warm days, the non-irrigated green roof tended to exhibit even higher surface temperatures than a conventional gravel roof. Hydrologically, the smart irrigation system required lower amounts of irrigation compared to timer and sensor-based irrigation regimes (reduction by 46.3% and 23.5%, respectively) without negatively affecting plant vitality and showed notably better average runoff reduction performance for heavy precipitation events (86.3% vs. 68.4% and 79.5%) This study demonstrates that novel smart irrigation routines for extensive green roofs have the potential to enhance the contribution to urban microclimate regulation and sustainable water management.