• Written informed consent form (ICF) prior to beginning specific protocol procedures.

• Female or male patients ≥ 18 years of age.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.

• Histologically confirmed TNBC as defined by the most ASCO/CAP guidelines based on local laboratory results.

• Note: TNBC means tumors that have \<1 percent expression of Estrogen Receptor (ER) and Progesterone Receptor (PR) as determined by immunohistochemistry (IHC), and that are, for HER2, either 0 to 1+ by IHC, or IHC 2+ and fluorescence in situ hybridization (FISH) negative.

• Tumor size between \> 10 and 20 mm by mammogram and/or ultrasound, or ≤ 25 mm after biopsy by breast magnetic resonance imaging (MRI) as per local assessment.

• Note: Up to 25 mm of diameter using breast MRI is allowed if the MRI was performed within 2 weeks after the breast biopsy (due to tissue inflammation after the procedure).

• Node-negative status by clinical exam and local radiological evaluation.

• Bilateral tumors and/or multi-focal (e.g, 2, separate lesions in the same quadrant)/multi-centric (e.g, 2 separate lesions in different quadrants) tumors are allowed. The tumor with the most advanced T stage should be used to assess the eligibility and TNBC needs to be confirmed for each breast/focus. In these cases, both axillae need to be assessed for nodal involvement confirmation.

• No evidence of metastatic disease based on radiological assessment according to institutional practices.

• No previous definitive ipsilateral breast surgery for the current breast cancer.

⁃ No prior chemotherapy, targeted therapy, and/or radiation therapy with therapeutic intent for this cancer.

⁃ Willingness to provide tumor tissue and blood samples at baseline and at surgery.

⁃ Females of childbearing potential must have a negative urine or serum pregnancy test and be willing to use an adequate method of contraception according to study protocol during treatment and for at least 4 months after the last dose of pembrolizumab. Female patients must refrain from egg cell donation and breastfeeding during treatment with pembrolizumab and for at least 4 months after the last dose of pembrolizumab.

⁃ Male patients and female patients of childbearing potential who engage in heterosexual intercourse must agree to use institution specified method(s) of contraception and must refrain from donating sperm or eggs during treatment with pembrolizumab and for at least 4 months after the last dose of pembrolizumab.

⁃ Patient has adequate bone marrow, liver, and renal function:

∙ Hematological: White blood cell (WBC) count \> 3.0 x 10 9/L, absolute neutrophil count (ANC) ≥ 1.5 x 10 9/L, platelet count ≥ 100.0 x10 9/L, and hemoglobin ≥ 9.0 g/dL (≥ 5.6 mmol/L).

‣ Hepatic: total bilirubin ≤ institutional upper limit of normal (ULN) (except for Gilbert's syndrome); aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 2.5 times ULN.

‣ Renal: serum creatinine ≤ 1.5 x ULN or creatinine clearance ≥ 50 mL/min/1.73 m2 for patients with creatinine levels above institutional normal.

⁃ Patient must be accessible for treatment and follow-up.