An original sodium alginate from Tunisian seaweed (Cystoseira barbata) was purified and characterized by circular dichroism (CD) and ATR-FTIR spectroscopies. ATR-FTIR spectrum of C. barbata sodiumalginate (CBSA) showed the characteristic bands of mannuronic (M) and guluronic acids (G). The M/Gratio was estimated by CD (M/G = 0.59) indicating that CBSA was composed of 37% mannuronic acid and63% guluronic acid. The analysis of viscosity of CBSA showed evidence of pseudoplastic fluid behaviour.The emulsifying capacity of CBSA was evaluated at different concentrations (0.25–3%), temperatures(25–100◦C) and pH (3.0–11.0). Compared to most commercial emulsifiers, the emulsion formulatedby CBSA was found to be less sensitive to temperature changes and more stable at acidic pH. CBSAwas examined for antioxidant properties using various antioxidant assays. CBSA exhibited importantDPPH radical-scavenging activity (74% inhibition at a concentration of 0.5 mg/ml) and considerable ferricreducing potential. Effective hydroxyl-radical scavenging activity (82% at a concentration of 5 mg/ml) andpotent protection activity against DNA breakage were also recorded for CBSA. However, in the linoleate--carotene system, CBSA exerted moderate antioxidant activity (60% at a concentration of 1.5 mg/ml).Therefore, CBSA can be used as a natural ingredient in food industry or in the pharmaceutical field.