A novel polysaccharide (GPP-S), with a molecular mass of 1.2 × 10(6) Da, was isolated from the tetraploid Gynostemma pentaphyllum Makino by alkali extraction followed by purifications using DEAE and Sephacryl S-400 column chromatographies. The monosaccharide composition of GPP-S was determined as rhamnose, arabinose, glucose, and galactose with a molar ratio of 1.00:3.72:19.49:7.82. The structural analysis suggested that the backbone of GPP-S is (1→4)-linked-glucose and (1→6)-linked-galactose with a (1→4,6)-linked-glucose branch every six monosaccharide residues. The terminals were 1-)-α-arabinose, glucuronic acid, and other monosaccharides. GPP-S exhibited scavenging capacities against hydroxyl, peroxyl, and DPPH(•) radicals in vitro. GPP-S also had inhibitory activities on IL-1β, IL-6, and COX-2 gene expressions in RAW 264.7 mouse macrophage cells. These results suggested that GPP-S could be developed as a bioactive ingredient for functional foods and dietary supplements.