BACKGROUND The association of radiotherapy with breast cancer survival in patients who underwent a mastectomy and had micrometastases in the sentinel lymph node is unclear. MATERIAL AND METHODS The survival benefit of radiotherapy was examined in patients with T0/1-T2N1mi breast cancer undergoing mastectomy plus sentinel lymph node biopsy (SLNB). Kaplan-Meier curves were employed for survival analysis and competing risk analysis, and a propensity score matching (PSM) cohort was enrolled to investigate whether such patients benefit from radiotherapy. RESULTS We identified 2864 patients in the SEER database from 2004 to 2015. All eligible patients were divided into the radiotherapy and the no-radiotherapy cohorts. With the median follow-up of 53 months, 5-year breast cancer-specific survival (BCSS) was 94.4% vs 95.2% (P=0.135), and 5-year overall survival (OS) was 91.2% vs 90.1% (P=0.466) in the radiotherapy cohorts and no-radiotherapy cohorts, respectively. The results of the competing risk analysis showed a comparable 5-year cumulative incidence of breast cancer-specific death (BCSD) in the radiotherapy and no-radiotherapy groups (5.5% vs 4.7%, P=0.107) but a higher 5-year cumulative incidence of other causes of death (OCD) in the no-radiotherapy cohort (3.3% vs 5.3%, P=0.011). No significant difference was observed for BCSS or OS in the PSM cohort. CONCLUSIONS Radiotherapy has no benefit for patients with T0/1-T2 breast cancer undergoing mastectomy with N1mi disease on SLNB. This analysis provides evidence that radiotherapy may safely be omitted in this group of patients.