Background: Dexamethasone (DEX) is a common medicine that is capable of causing malformation in the male reproductive system. The aim of this study was to investigate the effect of vitamin C (Vit-C) on spermatogenesis indexes and daily sperm production (DSP) in adult mice treated with DEX. Methods: Male Naval Medical Research Institute (NMRI) mice were divided into four groups: Control, DEX (7 mg/kg/day), Vit-C (100 mg/kg/day), and DEX þVit-C and treated for 7 days with intraperitoneal injection. Results: A significant increase in the mean levels of serum and tissue malondialdehyde (MDA) and apoptosis of Leydig cells was found in the DEX group compared to the control group. Sperm motility, DSP, tubular differentiation index, meiotic index, spermatogenesis index, the mean number of spermatocytes, round and long spermatids, and Leydig cells, and also serum testosterone level decreased in the DEX group compared to the control group. The results of this study indicate that Vit-C can significantly prevent the adverse effects of DEX on the mean number of spermatocyte, spermatid, and Leydig cells, tubular differentiation, meiotic and spermatogenesis index, DSP, sperm motility, and the mean levels of serum MDA. Conclusion: In conclusion, our results showed that coadministration of Vit-C and DEX prevents the adverse effects of DEX on the spermatogenesis indexes and DSP.