The MEDSAL hydrogeological observatory is a free web-GIS application that provides real-time critical information on coastal aquifers. It was implemented in the framework of the MEDSAL research project and included data and information on coastal aquifers in 5 Mediterranean countries (Greece, Cyprus, Tunisia, Italy, Tunisia, and Turkey). It includes important functions such as real-time CTD sensor measurements, physicochemical analyses and generation of multiple thematic maps, early warning system of salinity in groundwater, interactive thematic maps, etc.)