Background: This study aimed to investigate the role of postoperative radiation therapy in a large populationbased cohort of patients with stage I???III male breast cancer (MaBC). Methods: Patients with stage I???III breast cancer treated with surgery were selected from the Surveillance, Epidemiology, and End Results cancer database from 2010 to 2015. Multivariate logistic regression identified the predictors of radiation therapy administration. Multivariate Cox regression model was used to evaluate the predictors of survival. Results: We identified 1321 patients. Age, stage, positive regional nodes, surgical procedure, and HER2 status were strong predictors of radiation therapy administration. There was no difference between patients who received radiation therapy and those who did not (P = 0.46); however, after propensity score matching, it was associated with improved OS (P = 0.04). In the multivariate analysis of the unmatched cohort, the factors associated with better OS were administration of radiation therapy and chemotherapy. In the subset analysis of the unmatched cohort, postoperative radiation therapy was associated with improved OS in men undergoing breast-conserving surgery (BCS), with four or more node-positive or larger primary tumours (T3/T4). Furthermore, we found no benefit of radiation therapy, regardless of the type of axillary surgery in mastectomy (MS). In older MaBC patients with T1-2N1 who underwent MS, radiation therapy showed no significant effects, regardless of chemotherapy. Conclusion: Postoperative radiation therapy could improve the survival of MaBC patients undergoing BCS, with four or more node-positive or larger primary tumours. Moreover, it should be carefully considered in patients undergoing MS and older T1-2N1 patients.