The oxidative stress, inflammation and apoptosis contribute to testicular dysfunction in patients with diabetes mellitus (DM). Accordingly, the present study aimed to evaluate the potential ameliorative effects of systemic administration of bone marrow mesenchymal stem cells (BM-MSCs) on testicular injury in prepubescent male rats with streptozotocin (STZ)-induced diabetes. A total of 24 four-week-old male albino rats were equally divided into four groups: rats administered normal saline only (control group), STZ-treated rats (diabetes group), STZ-treated rats administered 2×10⁶ BM-MSCs one week after STZ therapy (early MSC treatment group), and rats administered 2×10⁶ BM-MSCs four weeks after STZ injection (late MSC treatment group). At seven days after BM-MSC administration, serum and testicular tissue samples were obtained for biochemical, histological, and ultrastructural analyses. The results indicated that in contrast to the control group, diabetes group showed low levels of testosterone, follicle-stimulating hormone, and luteinizing hormone in sera, and decreased level of glutathione and diminished activities of superoxide dismutase, catalase, and glutathione peroxidase in testicular tissues. However, the total oxidant status, thiobarbituric acid reactive substances, tumor necrosis factor-α and interleukin-1β were significantly increased in diabetes group compared to the control one. Also, the histological examination demonstrated hypoplasia and arrest of spermatogenesis at various stages in diabetes group. Moreover, the cytoplasm of their spermatogenic cells was characterized by a high number of defective mitochondria and expanded rough endoplasmic reticula. On the other hand, the early administration of BM-MSCs ameliorated most of the effects of STZ-induced diabetes in testicular tissues. These findings demonstrate that early systemic administration of BM-MSCs to prepubescent rats can protect against testicular injury and may restore testicular function in a model of STZ-induced diabetes.