Brown algae are a valuable source of bioactive polysaccharides, particularly alginates and fucoidans, with promising applications in cosmetics and pharmaceuticals. In this study, polysaccharides were extracted from four brown macroalgae, Padina pavonica, Sargassum ilicifolium, S. latifolium, and Turbinaria decurrens, collected along the coast of Djibouti. The properties characterization of the extracts was performed using FT-IR and ¹H-NMR spectroscopy. The M/G ratio of alginates revealed species-specific variability, with the highest ratio recorded for P. pavonica and the lowest for T. decurrens. FT-IR spectra showed a slightly higher intensity of the band around 1025 cm⁻¹ (M blocks) compared to 1082 cm⁻¹ (G blocks), indicating a mannuronic acid-rich composition. A strong band around 1600 cm⁻¹ was also observed, and T. decurrens showed the highest degree of sulfation (83.34% transmittance). The biological activities of the fucoidan-rich extracts were evaluated through DPPH radical scavenging and FRAP antioxidant assays, as well as tyrosinase and elastase inhibition tests. Significant antioxidant activities were observed across all species. P. pavonica showed the highest elastase inhibition (63.19%), followed by S. ilicifolium and T. decurrens (60%), and S. latifolium (52%). In the tyrosinase assay, S. ilicifolium (62%) and P. pavonica (57%) demonstrated notable inhibition at 0.5 mg/mL. These results highlight the multifunctional potential of brown algal polysaccharides and their relevance for marine-based cosmetic applications. The observed structure–activity relationships emphasize the impact of subtle structural differences on biological efficacy, supporting further exploration of marine algae in sustainable biotechnology.