Diabetes mellitus (DM) induces the production of reactive oxygen species, which may lead to cell injury and death. This study aimed to assess the effects of rosmarinic acid (RA) on testicular damage, oxidative stress, and apoptosis in streptozotocin (STZ)-induced diabetic albino mice. DM in four- to six-week-old BALB/c male albino mice was induced via 50 mg/kg STZ, IP for 5 days. Twelve mice were randomly assigned into each of following groups: a control group, a diabetic (DM) group, RA5 mg/kg and RA15 mg/kg groups, and DM + RA5 mg/kg and DM + RA15 mg/kg groups. RA doses were intraperitoneally injected six times a week for seven weeks. Diabetes increased blood sugar and HbA1c levels and decreased all assessed sperm parameters. Testicular tissues of the diabetic mice showed increased lipid peroxidation, decreased reduced glutathione levels and catalase and superoxide dismutase activities, and increased apoptosis associated with histological abnormalities. Both RA doses had no effects on final body weight, blood sugar, and HbA1c in the diabetic mice. It is concluded that the administration of the potent antioxidant RA to diabetic mice improved the redox status in testicular tissues, protected them from diabetes-induced oxidative damage, and improved the quality of spermatozoa, mostly in a dose-dependent manner, which suggests a potential application value of RA in treating DM-related testicular injury and perhaps other complications.