Background: American Joint Committee on Cancer (AJCC) 8th Edition Breast Cancer Staging System, biological information in addition to anatomical information was added as a basis for prognosis evaluation, but its prognostic impact in Japanese patients treated with postoperative radiotherapy has not been discussed. To determine the applicability of the updated prognostic staging system, we evaluated the prognostic value and confirmed the effectiveness of this system in patients in whom postoperative radiotherapy was performed. Materials and methods: Patients who were treated with postoperative radiotherapy for breast cancer in our institution between January 2011 and December 2015 were restaged by the AJCC 8th pathological prognosis staging system, and then overall survival (OS), disease-free survival (DFS) rates and hazard ratios (HR) were analyzed to compare the predictive fit of the two staging systems. Results: Five hundred and seven patients who in whom postoperative radiotherapy was performed for breast cancer were enrolled in this study. 36.1% patients were downstaged and 5.3% patients were upstaged from the 7th to 8th editions staging system classification. Kaplan–Meier curves and HRs showed differences in OS and DFS rates between the 7th edition and 8th edition staging systems. The AJCC 8th edition prognostic stage system has a better prognostic prediction of OS and DFS than does the 7th edition anatomic stage system. Conclusions: Compared with the 7th edition in breast cancer, AJCC 8th edition prognostic stage system has more precise stratification and superior prognostic value, providing a more accurate reference for the choice of radiotherapy for patients with breast cancer.