Lycium barbarum L. (Solanaceae) is a Chinese medicinal plant whose fruits (Fructus Lycii) are used by Chinese physicians for treatment of infertility. However, the active ingredients and the mechanism of action underlying Lycium's fertility-facilitating effects remain unknown. Here we report that Fructus Lycii polysaccharides (FLPS) inhibit time- and hyperthermia-induced structural damage in murine seminiferous epithelium, in vitro. Moreover, we found that FLPS delayed apoptosis in this system, both at normothermic and hyperthermic culture conditions. Oxidative stress was reported to be a major cause of structural degradation and apoptosis in hyperthermic testes, and thus the protective effect of FLPS could implicate an antioxidant mechanism of action. To test this hypothesis we assayed the effect of FLPS on ultraviolet light-induced lipid peroxidation, and cytochrome c reduction by free radicals. We found that FLPS is a potent inhibitor of both of these reactions. Together, these results demonstrate the protective effect of FLPS on time- and hyperthermia-induced testicular degeneration in vitro, indicate the potential mechanism of action for this protective effect, and provide a scientific basis for the traditional use of this plant.