Sargassum is a genus of brown algae that causes a major impact to marine ecosystems. Although accumulation of this invasive seaweed is considered an environmental problem, Sargassum can also be resource of valuable molecules such as alginates. Alginates are polysaccharides extracted from the cell walls of brown seaweeds with multiple applications in food, cosmetics, pharmaceuticals, and biotechnology. In this study, we explored four different extraction protocols to isolate alginates from Sargassum from the archipelago of Guadeloupe. H-NMR, FTIR, and SEM were carried out to characterize isolated samples and describe their chemical and morphological properties. Antioxidant capacity of the isolated alginates was tested in the sample. FRAP assay showed free radical scavenging activity, and cell viability assay showed that alginates keep protective capacity against oxidative stress in cell cultures. In sum, the isolation methods used in this study are effective extractive methods to obtain alginates from Sargassum, which highlight this seaweed as a potential source of bioactive compounds that may be exploited for industrial and biomedical purposes.