Urban environmental resilience has become a critical priority in the context of rapid urbanization, climate change, and ecological degradation. This study examines the relationship between urban density and environmental resilience, with particular attention to the mediating role of green infrastructure. Guided by a conceptual framework grounded in ecological urbanism, the research identifies key indicators of urban density (built-up ratio, population density, land-use mix), green infrastructure (green space per capita, NDVI, accessibility to green areas), and environmental resilience (air quality, surface temperature, ecological pressure index). A comparative case study was conducted in two dense districts of Tehran—Districts 6 and 10—using satellite imagery, GIS-based spatial analysis, and structural modeling with SmartPLS. The findings reveal a significant negative correlation between high urban density and environmental resilience indicators, particularly in areas where green infrastructure is limited or unevenly distributed. Conversely, well-integrated and accessible green spaces were found to mitigate ecological pressures and strengthen local resilience, thereby serving as an effective mediating factor. These results underscore the importance of investing in spatially equitable green infrastructure to offset the adverse environmental impacts of urban density and to foster more sustainable and livable urban environments. Beyond its empirical contribution, this study provides policy recommendations for integrating green infrastructure into high-density urban planning and offers a replicable analytical model for assessing environmental resilience in metropolitan contexts.