<p>The rapid growth of urbanization and industrialization has boosted economic development, but also caused urban sprawl and environmental degradation. This decline in environmental quality threatens the sustainability of cities, not only in large metropolitan areas but also in mid-sized cities like Agartala. Issues such as pollution, loss of green spaces, and heat island effects harm human health and well-being. Therefore, evaluating urban environmental quality is crucial to identify critical areas of degradation and support sustainable urban planning and policy. This study aims to develop a Remote Sensing Ecological Index (RSEI) to assess the urban environmental quality of Agartala and analyze spatiotemporal changes from 2004 to 2024 for effective environmental management. Landsat images were obtained from USGS for 2004 (Landsat-5) and 2014 and 2024 (Landsat 8 OLI/TIRS) to assess urban environmental quality. The RSEI is derived from five key indicators such as NDVI, MNDWI, NDBI, NDBaI, and LST, which are integrated using principal component analysis. Findings reveal that the percentage of land under very poor environmental conditions increased from 0.049% (0.038&#xa0;km²) in 2004 to 0.096% (0.074&#xa0;km²) in 2014 and 0.500% (0.383&#xa0;km²) in 2024. Similarly, areas classified as poor RSEI rose from 3.077% (2.354&#xa0;km²) in 2004 to 7.994% (6.116&#xa0;km²) in 2014 and 14.360% (10.986&#xa0;km²) in 2024. The mean RSEI declined from 0.665 in 2004 to 0.579 in 2014 and further to 0.578 in 2024, indicating a consistent deterioration in environmental quality. Between 2004 and 2014, 45.73% (34.98&#xa0;km²) of the area degraded, 7.58% (5.80&#xa0;km²) improved, and 46.70% (35.73&#xa0;km²) remained unchanged. The findings provide valuable insights for planners and policymakers to develop sustainable strategies, promote eco-friendly urban environments, and preserve natural resources for future generations. The proposed RSEI methodology can also be adapted and applied to other cities worldwide.</p>