A water-soluble heteroglycan (PS-I) isolated from the aqueous extract of a wild edible mushroom Lentinus sajor-caju showed average molecular weight ∼1.79 × 10 5 Da. The structure of the polysaccharide was determined using chemical and 1D/2D NMR experiments. Acid hydrolysis indicated the presence of D-glucose, D-galactose, D-mannose, and L-fucose in a molar ratio of nearly 4:4:1:1 respectively. The presence of terminal Fucp, terminal Galp, (1 → 3)-Glcp, (1 → 6)-Galp, (1 → 6)-Glcp, (1 → 4,6)-Galp, and (1 → 2,4)-Manp moieties were established from methylation analysis. The chemical and NMR analyses indicated that the PS-I was a heteroglycan composed of a repeating unit with backbone chain of three (1 → 6)-α-D-galactopyranosyl residues, two (1 → 6)-β-D-glucopyranosyl residues, one (1 → 4)-α-D-mannopyranosyl residue, and two (1 → 3)-β-D-glucopyranosyl residues where one (1 → 6)-α-D-galactopyranosyl residue was branched at O-4 position with terminal α-L-fucopyranosyl residue and (1 → 4)-α-D-mannopyranosyl residue was branched at O-2 position with terminal α-D-galactopyranosyl residue and the structure was proposed as; The PS-I is a moderate antioxidant compound which showed DPPH radical scavenging activity, hydroxyl radical scavenging activity, ABTS radical scavenging property, reducing power, and ferrous ion chelating ability. [Figure presented] © 2017 Elsevier B.V.