The River San Juan (RSJ), in the border between Nicaragua and Costa Rica, is one the major rivers in Central America and drains the largest basin in the region (38570 km²) in terms of volume. Extending from Lago Cicibolca to the Caribbean Sea, the RSJ is an important source of freshwater, sediments, nutrients and pollutants to the continental shelf. Ecosystems degradation, contamination of water bodies and overexploitation of natural resources, particularly deforestation in the southern part of the basin, modified and incremented sedimentation processes in the basin and the coastal margin during the last few decades. Advanced Very High Resolution Radiometer and Sea-viewing Wide Field-of-view Sensor data, together with in-situ CTD, current meter, turbidity sensor and water sampling for chemical analysis data are used to study, for the first time, the spatial and temporal variability of the plume of the RSJ and the coastal zone under its influence. An atmospheric correction has been implemented in order to obtain water reflectance from the AVHRR channels 1 and 2. Results from the above satellite and in-situ observations are presented