The aim of the present study was to investigate whether oxidative stress and immune dysfunction could be attenuated by Ganoderma atrum polysaccharide (PSG-1) in d-galactose (d-gal)-induced aging mice, and provide evidence for its effects. The results showed that PSG-1 significantly decreased lipid peroxidation in liver, brain, and spleen, but concomitantly increased the activities of superoxide dismutase, catalase, and glutathione peroxidase compared with the d-gal group. Elevation of glutathione contents and attenuation of glutathione disulfide contents were also found in PSG-1-treated animals. Furthermore, the results showed that PSG-1 treatment increased basal lymphocyte proliferation as well as T cell and B cell proliferation and enhanced interleukin-2 production. Taken together, the results suggested that PSG-1 had potential as a novel agent to promote health and improve aging-associated pathologies, at least in part, via modification of the redox system and improvement of immune function.