A water soluble branched β-D-glucan (PS-I) with an average molecular weight ∼2.1×10(5)Da was isolated from alkaline extract of the fruit bodies of the edible mushroom Entoloma lividoalbum (Kühner & Romagn) Kubička which consists of terminal β-D-glucopyranosyl, (1→3)-β-D-glucopyranosyl, (1→6)-β-D-glucopyranosyl, and (1→3,6)-β-D-glucopyranosyl moieties in a molar ratio of nearly 1:3:2:1. The structure of PS-I was elucidated using acid hydrolysis, methylation analysis, periodate oxidation study, partial hydrolysis, and 1D/2D NMR experiments. The repeating unit of the polysaccharide (PS-I) contains a backbone chain of three (1→6)-β-D-glucopyranosyl residues, one of which was branched at O-3 position with the side chain consisting of three (1→3)-β-D-glucopyranosyl and a terminal β-D-glucopyranosyl residues. Total antioxidant capacity of 1mg PS-I was measured and found equivalent to 70±15μg of ascorbic acid. The PS-I was found to possess hydroxyl and superoxide radical-scavenging activities with EC50 values of 480 and 150μg/mL, respectively. The reducing power of PS-I was determined 0.5 at 480μg/mL.