ABSTRACT Ecological environmental quality (EEQ) directly influences public health, resource availability, and climate resilience for national ecological security and sustainable development. Urban agglomerations experience growing ecological stress under rapid urbanization, yet integrated assessments of EEQ dynamics remain limited. This study develops a Novel Remote Sensing Ecological Index (NRSEI) on the Google Earth Engine (GEE) platform by integrating multi‐source datasets, including Landsat imagery and NPP–VIIRS nighttime light data. The Yangtze River Delta Urban Agglomeration (YRDUA) is selected as the study area to analyze long‐term variations in EEQ. The NRSEI integrates five core indicators, namely vegetation greenness, surface wetness, land surface temperature, air pollution represented by PM 2.5 , and human activity intensity, providing a comprehensive assessment of EEQ. Results from 2003 to 2023 reveal an “N‐shaped” temporal pattern: slight improvement (2003–2008), decline (2008–2018), and moderate recovery (2018–2023). Spatially, EEQ exhibits a clear northwest–southeast gradient, with lower values in industrial–agricultural zones and higher values in mountainous and coastal areas. Geographical Detector analysis identifies elevation, mean temperature, and economic development as dominant drivers of EEQ heterogeneity, with synergistic effects exceeding individual influences. Overall, this study establishes a refined and scalable framework for long‐term, high‐resolution ecological monitoring and provides empirical evidence to guide balanced urban and ecological development in rapidly urbanizing regions.