The aim of this work is to develop new organic bio-sourced inhibitors that are ecofriendly and biodegradable. These natural inhibitors are organic, non-toxic molecules derived from plant extracts, containing numerous secondary metabolites, and are capable of being highly effective in protecting metals against corrosion. This study concerns the extraction, characterization and electrochemical study of natural organic compounds extracted from a species of Sargassum abundant on the coasts of Martinique, Sargassum fluitans III. The objective is to inhibit the corrosion of carbon steel. Electrochemical impedance spectroscopy (EIS) and linear polarization (LP) techniques have led to new experimental results showing inhibitor efficacy. The results obtained show that this plant extract could serve as an effective inhibitor for the C38 steel in acidic media. Studies on the phytochemicals of the crude extract were also carried out. Electrochemical studies, on each chemical families present, were also established to find the main constituents responsible for corrosion inhibition properties of the algae extract. The adsorption of Sargassum fluitans III extract on the C38 steel surface, obeys the Langmuir adsorption isotherm.