To obtain a low-molecular-weight polysaccharide with immuno-enhancing activity, hot water extract of Ganoderma lucidum fruit bodies was separated by membrane ultrafiltration, anion exchange, and gel filtration chromatography, and the immunological activities of fractions were assessed on the basis of nitric oxide production by RAW 264.7 macrophages. A novel polysaccharide (TB3-2-2) was successfully isolated and purified. TB3-2-2 is a homogeneous polysaccharide, with a relative molecular weight of 5.11 × 103 Da, identified by high-performance liquid chromatography and was composed of galactose and glucose in a molar ratio of 2:3 determined by high-performance anion exchange chromatography. TB3-2-2 had a carbohydrate content of 99%, as measured using the phenol-sulfuric acid method. Proliferation of mouse spleen lymphocytes and the expression level of interleukin-6 was significantly increased by TB3-2-2. Results indicate that the low-molecular-weight polysaccharide with immunological activity in G. lucidum is worthy of further research and development.