A purified single polysaccharide (HAP-I) that was isolated from the fruit bodies of Russula vinosa was used as the study object, and its molecular weight distribution and purity were determined based on the separation results from gel permeation chromatography (GPC) with dextran-based Sephadex G-100. The chemical structure of the HAP-I molecule was studied by Fourier transform infrared spectroscopy (FT-IR), gas chromatography (GC), periodate oxidation, Smith degradation, and nuclear magnetic resonance (NMR). The antioxidant activity of HAP-I was evaluated by measuring its reducing power and in vitro scavenging activities on 1,1-diphenyl-2-picrylhydrazyl (DPPH), superoxide anion (O2-), and hydroxyl (OH) free radicals. The results revealed that the weight-average relative molecular mass (MW) of HAP-I was 16,861; FT-IR spectra indicated the presence of characteristic absorption peaks of polysaccharides and an amino-group-containing β-D-glucose ring structure in HAP-I. GC results showed that HAP-I contained four monosaccharides, rhamnose (Rha), mannose (Man), glucose (Glu), and galactose (Gal), with a molar ratio of 1:8.29:7.45:18.13. The results from periodate oxidation and Smith degradation revealed that HAP-I had a D-glucopyranose ring containing mannosidase, including α- and β-configurations; the branches of HAP-I were composed of 66.7% (1→3) glycosidic linkages, 22.24% (1→6) and (1→) sugar residues, and 11.06% (1→2) sugar residues (without (1→4) glycosidic linkage). NMR results showed that HAP-I contained galactosyl pyranose with both α- and β-configurations. The in vitro antioxidant activity test suggested that HAP-I had strong scavenging activities on DPPH, O2-, and OH radicals, as well as strong reducing power.