Lycium ruthenicum Murr. is commonly used in traditional Tibetan medicine, and the fruits of Lycium ruthenicum Murr. contain an immunologically active pectin which improves immune function against chronic diseases. The present study was performed to evaluate the immunomodulatory effects of Lycium ruthenicum Murr. polysaccharide 3 (LRGP3) in cyclophosphamide (Cy)-induced immunosuppressed mice. Mice were injected intraperitoneally once daily with low-dose (25 mg/kg), intermediate-dose (50 mg/kg), high-dose (100 mg/kg) of LRGP3 for 10 consecutive days, respectively. Compared with Cy group, LRGP3 accelerated recovery of spleen and thymus indices, enhanced T cell and B cell proliferation responses, as well as peritoneal macrophage phagocytosis. In addition, LRGP3 treatment restored the levels of interleukin-2 (IL-2), IL-6 and tumor necrosis factor-α (TNF-α) in the serum of Cy-treated mice. These results indicate that LRGP3 plays an important role in the protection against immunosuppression in Cy-treated mice and could be a potential immunomodulatory agent.