This study aims to quantitatively assess the performance of Tehran's existing urban green infrastructure in regulating runoff and to economically value this ecosystem service using an integrated framework combining remote sensing, Geographic Information Systems (GIS), the SCS-CN (Soil Conservation Service Curve Number) model, and economic valuation methods. Environmental indices—including the Green Infrastructure Index, Normalized Difference Vegetation Index (NDVI), Normalized Difference Built-up Index (NDBI), and land-use data—were derived from remote sensing data. Subsequently, runoff volume under current urban conditions was estimated using the SCS-CN model, and the role of green infrastructure in runoff reduction was quantified by comparing the existing scenario against a hypothetical scenario excluding the effects of green infrastructure. The results indicated that the mean Green Infrastructure Index for Tehran is 0.271. The runoff volume generated under current conditions was calculated at 26.59 million cubic meters, while the existing green infrastructure successfully regulated approximately 1.47 million cubic meters of runoff. The economic value of this ecosystem service was estimated at approximately 2.34 million USD. The results indicate that Tehran's urban green infrastructure, in addition to its environmental role, possesses measurable hydrological and economic functions and can serve as an effective strategy for sustainable runoff management and enhanced urban resilience.