Hydrocolloids from seaweeds have interesting functional properties, such as antioxidant activity and gelling ability. A polysaccharide was isolated by aqueous extraction at 90 C from the red seaweed Gracilaria birdiae (Gb), with a yield of 27.2% of the seaweed dry weight. The sulfate content of the polysaccharide was 8.4% and the main sugars present were galactose (65.4 mol%), 3,6-anhydrogalactose (25.1 mol%) and 6-O-methylgalactose (9.2 mol%). Gel permeation chromatography showed that Gb polysaccharide is a heterogeneous system, with molar mass at the main peak of 3.7 Â 10 5 g mol À1 and a shoulder of 2.6 Â 10 6 g mol À1 . The sulfated polysaccharide of Gb characterized by FTIR exhibits the characteristic bands of agarocolloids (at 1375 and 770 cm À1). The rheological behavior of Gb sulfated polysaccharide exhibits a gel-like behavior close to the one observed in commercial agar. The antioxidant properties of Gb sulfated polysaccharide were evaluated by measuring DPPH free-radical scavenging effect, showing that this polysaccharide has a moderate effect in inhibiting the formation of those radicals.