Purpose: Randomized trials including ACOSOG Z0011 and SENOMAC mostly included patients with hormone receptor-positive breast cancer, but a limited number of patients with aggressive tumor biology. Therefore, we assessed the oncological safety of omitting axillary dissection in patients with SLN-positive HER2-positive or triple-negative breast cancer at upfront surgery. Materials and Methods: This retrospective cohort study included patients with clinically node-negative HER2-positive and triple-negative breast cancer who had sentinel lymph node biopsy (SLNB) alone with pN+ disease. Almost all patients (97.5%) received nodal irradiation. Results: Between 2015 and 2020, 118 patients with HER2-positive (n = 79, 67%) and triple-negative (n = 39, 33%) tumors were included in the study from 8 centers. Of those, 94.9% were cT1-2 and 72% underwent breast-conserving surgery. Most patients (n = 98, 83.1%) had 1 metastatic sentinel lymph node. Among those with involved sentinel lymph nodes, 59 (50%) had macrometastasis, 43 (36.4%) had micrometastasis and 16 (13.6%) had isolated tumor cells. After a median follow-up of 53 months, the locoregional recurrence rate was 2.5% without any axillary recurrence, and systemic recurrence rate was 11.9%. Factors associated with worse disease-free survival were having a cT2-3 stage and a triple-negative subtype disease. Having triple-negative tumor was the only significant factor associated with worse disease-specific survival. Conclusion: Patients with cN0 HER2-positive and triple-negative breast cancer with low-volume axillary metastases treated with upfront SLNB-alone showed excellent local control with nodal irradiation.