The efficacy of adjuvant radiotherapy for the treatment of triple negative breast cancer patients with varying numbers of positive lymph nodes is not clear. We assessed the association between adjuvant radiotherapy and survival in 943 T1/T2 triple negative breast cancer patients treated at our institute between 2008 and 2012. We determined that post-operative radiotherapy improved overall survival (OS), disease-free survival (DFS), and local recurrence-free survival (LRFS) in patients with ≥ 4 positive nodes (p = 0.037, p = 0.035, and p = 0.012, respectively). Although Cox regression analysis demonstrated that radiotherapy was a significant prognostic factor in triple negative breast cancer with ≥ 4 positive nodes, post-operative radiotherapy had no clear effect on OS, DFS, or LRFS in patients with 1-3 positive nodes (p = 0.849, p = 0.860, and p = 0.162, respectively). The prognosis (i.e., OS, DFS, and LRFS) of triple negative breast cancer patients without lymph node metastasis who underwent breast-conserving surgery and post-operative radiotherapy was similar to that of patients who underwent mastectomy alone (p = 0.336, p = 0.537, and p = 0.978, respectively). Our findings demonstrate that post-operative radiotherapy is beneficial for T1/T2 triple negative breast cancer patients with ≥ 4 positive lymph nodes.