Seaweeds are widely used as food and natural remedies among the Asian population. In this study, the biochemical composition of Cystoseeira amentacea, Cystoseira sedoides, and Sargassum vulgare from the Algerian coast was evaluated. The results indicated that the protein content ranged from 8.62–9.56%, whereas the lipid content varied from 0.77 to 2.35%. The most prevalent unsaturated fatty acid was oleic acid (14.47–18.64%). The seaweeds contain phenolic compounds in varying amounts, LC-MS-MS analysis showed the presence of catechin, p-coumaric acid, caffeic acid, and gallic acid. Moreover, the phaeophyta are rich in potassium and calcium. However, they accumulate high quantities of heavy metals including arsenic (403.92 ± 1.299 to 508.62 ± 1.364 mg/100g of dry matter). The results suggest that these algae are rich in primary and secondary metabolites, however, they cannot serve as human food, due to their toxicity. Consequently, they can used as bioindicators of marine pollution. Keyworlds : Seaweeds, Phenolic compounds, Fatty acids, Proteins, Flavonoids, Heavy metals.