Since 2011, massive strandings of Sargassum (brown alga) have significantly affected Caribbean islands causing major health, environmental and economic problems. Amongst them, the degradation of algae releases corrosive gases, hydrogen sulphide (H2S) and ammonia (NH3) which causes an accelerated corrosion of the metallic structures of these coastal areas. The aim of this study was to quantify the impact of Sargassum strandings on the corrosion of three types of steels (DC01 carbon steel, 304L and 316L stainless steels) immersed for up to 120 days at various sites in Martinique which were gradually impacted by Sargassum. A multidisciplinary approach was developed, incorporating: (i) surface analysis through macrophotography and corrosion product examination, (ii) weight loss measurements, and (iii) analysis of physicochemical parameters alongside microbial composition. As a result, in the presence of degraded Sargassum, an anaerobic, reducing and more acidic environment was correlated with high corrosion rates for all studied steels. When high density of Sargassum sp. was present, elemental sulphur was identified in the corrosion product layers of DC01 and 316L. Moreover, in this condition, sulphate-reducing bacteria (SRB) were observed in the surface biofilms of 304L coupons such as Desulfobulbus rhabdoformis. All these factors have highlighted the aggressiveness of the medium resulting from the presence of decomposing Sargassum, leading to increased corrosion rates. Our work provides new information on the importance of managing Sargassum strandings in order to avoid accelerated degradation of metallic structures in harbours and coastal zones.