In this study, the extraction of water-soluble polysaccharides from the seed coat of black soybean (BSCP) was investigated and optimized. A response surface methodology based on a Box-Behnken design (BBD) was used to optimize the extraction conditions as follows: extraction temperature, 100°C; ratio of water to material, 22.3 mL/g; and extraction time, 133.2 min. Under these conditions, the experimental yield of polysaccharides was 10.56%, which was consistent with the predictive yield. A novel galactomannan, BSCP-1, with a molecular weight of 7.55 × 105 Da determined by high-performance gel permeation chromatography, was isolated from the black soybean seed coat. Through gas chromatography-mass spectrometry analysis, BSCP-1 was identified as a galactomannan consisting of galactose, mannose and rhamnose in a molar ratio of 6.01:3.56:1.00. Cytotoxicity against the human gastric carcinoma cancer cell line was also determined.