Purpose: Immunotherapies targeting PD-1/L1 enhance pathologic complete response (pCR) rates when added to standard neoadjuvant chemotherapy (NAC) regimens in early-stage triple-negative, and possibly high-risk estrogen receptor–positive breast cancer. However, immunotherapy has been associated with significant toxicity, and most patients treated with NAC do not require immunotherapy to achieve pCR. Biomarkers discerning patients benefitting from the addition of immunotherapy from those who would achieve pCR to NAC alone are clearly needed. In this study, we tested the ability of MHC-II expression on tumor cells, to predict immunotherapy-specific benefit in the neoadjuvant breast cancer setting. Patients and Methods: This was a retrospective tissue-based analysis of 3 cohorts of patients with breast cancer: (i) primary nonimmunotherapy-treated breast cancers (n ¼ 381), (ii) triple-negative breast cancers (TNBC) treated with durvalumab and standard NAC (n ¼ 48), and (iii) HER2-negative patients treated with standard NAC (n ¼ 87) or NAC and pembrolizumab (n ¼ 66). Results: HLA-DR positivity on ≥5% of tumor cells, defined a priori, was observed in 10% and 15% of primary non-immunotherapy–treated hormone receptor–positive and triple-negative breast cancers, respectively. Quantitative assessment of MHC-II on tumor cells was predictive of durvalumab þ NAC and pembrolizumab þ NAC (ROC AUC, 0.71; P ¼ 0.01 and AUC, 0.73; P ¼ 0.001, respectively), but not NAC alone (AUC, 0.5; P ¼ 0.99). Conclusions: Tumor-specific MHC-II has a strong candidacy as a specific biomarker of anti–PD-1/L1 immunotherapy benefit when added to standard NAC in HER2-negative breast cancer. Combined with previous studies in melanoma, MHC-II has the potential to be a pan-cancer biomarker. Validation is warranted in existing and future phase II/III clinical trials in this setting.