Groundwater quality in coastal regions is a critical environmental and socio-economic issue, with salinization being a significant concern. This study examined the impact of sea level rise (SLR) and land use changes on groundwater quality in a coastal sub-watershed in Thiruvallur district, Tamil Nadu, India, from 2000 to 2020. Groundwater samples were collected monthly from September 2019 to January 2021, and analysed for electrical conductivity (EC), a key indicator of salinity. This data was supplemented with historical data (2000–2019) on rainfall and groundwater quality obtained from the State Ground and Surface Water Resources Data Centre, PWD, Tharamani, Tamil Nadu, India, enabling a comprehensive temporal analysis. Land use/land cover (LULC) maps were generated using Landsat/Sentinel satellite imagery to assess the influence of land use changes on groundwater quality. These maps, created in ArcGIS 10.2, focused on prominent land classes viz. water bodies, dense vegetation, scrubland, settlement, aquaculture and barren land. The study found that EC levels fluctuated over the study period, with areas of high EC (above 3500 µS/cm) decreasing from 73.93 km2 in 2000 to 38.09 km2 in 2020. The study also found that SLR increased from 0.81 cm in 2000 to 6.7 cm in 2020, suggesting a potential driver of seawater intrusion into coastal aquifers. The study concluded that the synergistic effects of SLR and groundwater over-exploitation contributed to elevated salinity levels within the study area. These findings emphasize the urgent need for sustainable groundwater management strategies and adaptation measures to mitigate the adverse impacts on groundwater resources.