In the Mediterranean Sea, seaweed marine forests, one of the most important benthic assemblages for the coastal ecosystem functioning, are shaping the rocky reefs. Among the Fucales, Gongolaria barbata (syn. Cystoseira barbata) is a several years living species usually growing in very shallow, sheltered and well-lit marine reef habitats and in coastal lagoons. The long-term change in its distribution in relation to a variety of disturbances has been assessed along the French Mediterranean coast, including Corsica and brackish lagoons, thanks to historical data dating back to the 19th century. The current distribution was established through an extensive survey conducted by snorkelling, encompassing approximately 3 000 km of coastline. A GIS analysis indicates that Gongolaria barbata can currently be considered as regionally extinct in French Catalonia and Western Provence and functionally extinct in the French Riviera. In Languedoc, the species is extinct in the open sea and only present in certain brackish lagoons despite severe repeated anoxic crises (malaïgues) and competition with a rich exotic flora introduced from the NE Atlantic and NW Pacific, especially the invasive Sargassum muticum. In contrast, the populations of G. barbata have remained stable in Eastern Provence and Corsica. The main possible causes of decline are uprooting, overgrazing by herbivores, habitat destruction and competition with introduced seaweeds. The relevance of ecological restoration of G. barbata populations was assessed.