<p>Natural antioxidants obtained from marine resources such as macroalgae&#xa0;should be studied in order to develop safer and more sustainable alternatives to synthetic compounds used in the food and pharmaceutical industry. Furthermore, multi-assay assessments contribute to a more thorough knowledge of antioxidant mechanisms, allowing researchers to identify bioactive fractions with focused potential for nutraceutical and medicinal applications. <i>Bostrychia scorpioides</i> (Hundson) Montagne 1842 is a red macroalga collected from the Oualidia lagoon (Moroccan atlantic coast). Its crude ethanolic extract, semi-purified and purified fractions of phycobiliproteins and sulfated galactans were evaluated for antioxidant activity using four complementary in vitro assays: nitric oxide (NO⁻) scavenging, hydrogen peroxide (H₂O₂) scavenging, lipid peroxidation inhibition, and β-carotene bleaching inhibition. The phycobiliproteins crude extract exhibited broad-spectrum antioxidant activity (93.92% ± 0.06), comparable to Trolox in the lipid peroxidation assay (83.47% ± 0.05). Notably, the second purified phycobiliprotein fraction showed exceptionnel NO scavenging activity (86.01% ± 0.12), surpassing the positive control Trolox (69.31% ± 0.01). In contrast, H₂O₂ scavenging was generally weaker in all the different samples. Overall, crude and semi-purified extracts had a stable, more broad-spectrum antioxidant effect likely due to synergistic interactions, unlike purified fractions with decreased or selective activity. These results highlight the importance of multi-assay profiling and the potential of <i>B. scorpioides</i> fractions for targeted nutraceutical and biomedical applications.</p> Graphical abstract <p>Graphical abstract illustrates the comprehensive experimental workflow applied to Bostrychia scorpioides, including the extraction, purification, and characterization of its bioactive compounds, as well as an evaluation of their antioxidant mechanisms</p>