Purpose This study was conducted to investigate the prognostic value of lymph node ratio (LNR) in stage II/III breast cancer patients who undergo mastectomy after neoadjuvant chemotherapy. Materials and Methods Clinical and pathological data describing stage II/III breast cancer patients were included in this retrospective study. The primary outcomes were locoregional recurrence-free survival (LRFS), distant metastasis-free survival (DMFS), disease-free survival (DFS), and overall survival (OS). Results Among 277 patients, there were 43 ypN0, 64 ypN1, 89 ypN2, and 81 ypN3 cases. Additionally, there were 43, 57, 92 and 85 cases in the LNR 0, 0.01-0.20, 0.21-0.65, and > 0.65 groups, respectively. The median follow-up was 49.5 months. Univariate analysis showed that both ypN stage and LNR were prognostic factors of LRFS, DMFS, DFS, and OS (p < 0.05). Multivariate analysis showed that LNR was an independent prognostic factor of LRFS, DMFS, DFS, and OS (p < 0.05), while ypN stage had no effect on prognosis (p > 0.05). Conclusion The integrated use of LNR and ypN may be suitable for evaluation the prognosis of stage II/III breast cancer patients who undergo mastectomy after neoadjuvant chemotherapy.