Résumé : Spatio-temporal changes during the last twelve year period (2006–2017) and their impacts on ecological and socio-economic status of Dondra lagoon, southern coast of Sri Lanka were studied as many lagoons in southern Sri Lanka are being seriously affected due to anthropogenic pressure in the recent past. The changes of Dondra lagoon and its immediate surroundings were studied in conjunction with a GIS-coupled ecological survey and a questionnaire survey. The lagoon water surface area has decreased by about 0.92 ha (~8%) and the mangrove cover has increased by about 1.38 ha (~11%) over this period. The salinity of the lagoon has also reduced, forming a ‘low saline’ (3 psu) regime. About 40% of the lost water surface has scarified for a newly formed land mass (~0.4 ha) within the proximal part of the lagoon. The bridge, broken by the tsunami of 2004, has newly been constructed twice during the reporting period. The construction most likely led to impair the inflow and outflow through the lagoon mouth. Several development projects were launched in the immediate periphery of the lagoon stimulated soil erosion causing heavy siltation in the lagoon. The above changes in the morphometry of the lagoon is a cumulative effect of two factors; impaired inflow and outflow through the lagoon mouth, and the increase of the sediment input to the lagoon. If the ongoing processes are sustained, the lagoon will change into a different landscape. Therefore, early intervention to restore the lagoon hydrology is highly recommended if the lagoon ecosystem is to be protected.