Abstract: Background and Objective: Restraint stress and hypothyroidism impair animal testicular functions. As stress is increasing gradually in our daily life, there is an increased chance of the occurrence of both stress and hypothyroidism together. Therefore, this study was designed to evaluate the effect of restraint stress on growth performance and testicular cells apoptosis in adult hypothyroid mice. Methodology: Twenty-four adult male mice were divided into four groups: control, Restraint Stress (RS), hypothyroid (HT) and RS+HT. ANOVA followed by Tukey as post hoc tests were used to determine the difference between multiple groups. Results: The results indicated that the feed and water index, body weight and testes weight, serum concentration of T3, T4 and testosterone exhibited a reduction but the TSH levels were increased in all experimental groups compared to the control. Histological observations of the testis from different experimental groups exhibited considerable interstitial edema, broken basement membrane and increased interstitial spaces compared with the control group. Seminiferous tubules were also morphologically shrinkage and deformed in RS+HT group compared to control group. Obvious suppression of spermatogenesis by restraint stress, the degenerative population of round spermatids was markedly increased due to apoptosis in the lumen of RS group as compared to the control group. Moreover, leydig cells, blood vessels, sertoli cells, primary and secondary spermatocytes are absent in large numbers and exposing apoptosis in RS+HT mice than RS and HT mice as compared to the control mice. Conclusion: Restraint stress and hypothyroidism together have adverse effects on male fertility based on increasing of the apoptotic process.