In this study, the antioxidant activities and biochemical characteristics of Ulva rigida (Ulvophyceae) and Arthrospira platensis (Cyanophyceae) were determined. The extracts from two seaweed species were evaluated for their free radical scavenging activity, using the 1,1-diphenyl-2-picrylhydrazyl hydrate (DPPH) method, their total phenolic, flavonoid, and condensed tannin contents, through Folin–Ciocalteu, Quettier-Deleu, and Price methods, respectively. The extracts of two seaweeds showed a low free radical scavenging capacity in comparison with commercial antioxidant BHT (butylated hydroxytoluene), and vitamin C. The extract of U. rigida demonstrated greater antioxidant potential with a low IC50 (3.76±0.02 mg/g Ext.). The total phenolic contents were ranged from 2.21±0.08 (U. rigida) to 8.59±0.62 (A. platensis) mg GAE/g of extract. The highest flavonoid content was found in A. platensis as 22.70±0.65 mg rutin/g of extract. The contents of condensed tannin were measured 3.01±0.11 mg CE/g of extract for A. platensis and 3.76±0.06 mg CE/g of extract for U. rigida. According to results obtained, U. rigida and A. maxima possess antioxidant activity and could be used in for future applications in medicine, functional foods, and agriculture.