Information on the characteristics of soybean polysaccharides is limited. This study was undertaken to purify and characterize a polysaccharide in mature soybean seeds (Glycine max (L.) Merr. cv. Akishirome). A soluble polysaccharide, A1-β, was extracted from the cotyledons of soybean seeds with water and purified by Bio-Gel A150m and DEAE-Sephacel column chromatography. The homogeneity of A1-β was confirmed by ultracentrifugal and electrophoretic analyses, and its molecular weight was ca. 2×106. The major components of A1-β were arabinose and galactose in the molar ratio of 2:3. Other components were rhamnose, fucose, xylose, mannose, glucose, mannuronic acid, galacturonic acid and glucuronic acid. These results indicate that A1-β may be a polysaccharide that has never been reported in soybean seeds.