Due to human and environmental interaction, Land Use and Land Cover are changing drastically. This study presents a comprehensive analysis of land-use and land-cover (LULC) changes in the Gal-Oya watershed, aiming to understand the spatial and temporal transformations over the past three decades. Using satellite imagery from 2005, 2015, and 2025, the study applied unsupervised classification techniques within a GIS framework to map and quantify changes across key land-cover categories, including forest, agriculture, built-up areas, and water bodies. Accuracy assessments and change detection analyses were conducted to ensure the reliability of the results. The results show that there are remarkable changes in all the land cover throughout the years. Paddy fields cover 565.63 ha of land; meanwhile, 96.07 ha of land were built on in 2005. The greatest and smallest changes in land cover were observed as 395.44 hectares and 142.52 hectares, respectively. However, in the current year 2025, 27.30% of areas cover 658.40 ha of dense forests, and 142.52 ha represent built-up areas, with 5.91% recorded as the highest and the least LULC changes, respectively. Barren lands changed into forest covers and scrublands, while cultivation areas were converted into scrublands and built-up areas. Other land use classes were also converted to other types in the Gal Oya region. Due to increasing population pressure, inadequate land management systems, and limited irrigation development, these changes have led to several ecological impacts and consequences in the study area. The findings of this study underscore the urgent need for a thorough evaluation of human activities within the Gal-Oya watershed and the implementation of sustainable land-use and land-cover practices, including closely monitoring and restoring bare lands, conserving forest and scrubland ecosystems, rehabilitating degraded and impoverished areas to create viable grazing lands, and curbing the unchecked expansion of cultivated zones. Such measures are crucial for mitigating environmental degradation and fostering long-term ecological resilience in the region.