Inclusion Criteria:

   - Age >= 18 years

   - Eastern Cooperative Oncology Group (ECOG) Performance Status 0-2

   - Triple Negative Breast Cancer:

      - Patients with a history of stage T1cN1-2 or T2-4N0-2 breast cancer according to
      the primary tumor-regional lymph node anatomic staging criteria of the American
      Joint Committee on Cancer (AJCC), 8th edition as determined by the investigator
      in radiologic assessment, clinical assessment or both

      - Patients must have no residual invasive disease in the breast or lymph nodes
      after the completion of neoadjuvant therapy. Residual ductal carcinoma in situ
      (DCIS) is allowed. Isolated tumor cells are considered node-negative

      - Estrogen (ER) and progesterone (PR) =< 10%; HER2-negative by American Society of
      Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines
      (immunohistochemistry [IHC] and fluorescence in situ hybridization [FISH])

      - If invasive disease was present in both breasts, participation in the study is
      permitted as long as the eligibility criteria are met for both tumors/breasts

   - Patients must have received neoadjuvant chemotherapy in combination with pembrolizumab
   for a minimum of 6 cycles. All systemic chemotherapy and ICI therapy should have been
   completed preoperatively

   - An interval of no more than 12 weeks between the completion date of the final surgery
   and the date of randomization

   * Note: Adjuvant radiation can be given on study. If given, it is encouraged to be
   given concurrently with pembrolizumab, per investigator discretion. Treatment with
   adjuvant pembrolizumab is strongly discouraged prior to participation in this trial,
   but if administered (e.g., if patients are awaiting pathology results), pembrolizumab
   may be administered for up to 6 weeks post-surgery and must be completed prior to
   registration

   - Use of investigational anti-cancer agents must be discontinued at time of registration

   - Adequate excision: Surgical removal of all clinically evident disease in the breast
   and lymph nodes as follows:

      - Breast surgery: Total mastectomy or breast-conserving surgery with histologically
      negative margins, including no ink on tumor for DCIS, at the time of excision

      ** For patients who undergo breast-conserving surgery, the margins of the
      resected specimen must be histologically free of ductal carcinoma in-situ (DCIS)
      as determined by the local pathologist. If pathologic examination demonstrates
      DCIS at the line of resection, additional operative procedures may be performed
      to obtain clear margins. If DCIS is still present at the resected margin after
      re-excision(s), the patient must undergo total mastectomy to be eligible.
      Patients with margins positive for classic lobular carcinoma in situ (LCIS) are
      eligible without additional resection

      - Lymph node surgery:

         - For a patient with clinically N0 disease, a sentinel lymph node biopsy
         should have been performed at time of surgical evaluation, and if
         pathologically node positive, the patient is no longer eligible. Isolated
         tumor cells are considered node-negative

         - For a patient with clinically N1 disease at diagnosis (with positive results
         from a fine-needle aspiration, core biopsy, or sentinel node biopsy
         performed prior to preoperative therapy) additional surgical evaluation of
         the axilla following preoperative therapy is required

         *** If they become cN0 (no palpable adenopathy), then a sentinel lymph node
         biopsy could have been performed at time of surgery (axillary dissection
         would also be permitted); if the sentinel lymph node biopsy is positive, the
         patient is no longer eligible

         - If sentinel node biopsy performed before preoperative therapy was negative,
         no additional surgical evaluation of the axilla is required after
         preoperative therapy. If sentinel node biopsy performed before preoperative
         therapy was positive, an ALND is required after preoperative therapy

         - If the only sentinel node identified by isotope scan is in the internal
         mammary chain, surgical evaluation of the axilla is still required

         - If sentinel node evaluation after preoperative therapy is negative, no
         further additional surgical evaluation of the axilla is required

         - Axillary dissection without sentinel node evaluation is permitted as the
         initial or sole axillary evaluation after preoperative therapy

   - If breast-conserving surgery was performed but patient will not be receiving breast
   radiation, the patient is not eligible

   - Not pregnant and not nursing, because this study involves an agent whose genotoxic,
   mutagenic and teratogenic effects on the developing fetus and newborn are unknown.
   Therefore, for women of childbearing potential only, a negative serum or urine
   pregnancy test done =< 7 days prior to randomization is required

   - Absolute neutrophil count (ANC) >= 1,000/mm^3

   - Platelet Count >= 100,000/mm^3

   - Estimated glomerular filtration rate (eGFR) >= 15 mL/min/1.73m^2

   - Total Bilirubin =<1.5 x upper limit of normal (ULN)

   * Patients with Gilbert's disease with a total bilirubin =< 2.5 x ULN and direct
   bilirubin within normal limits are permitted

   - Aspartate aminotransferase (AST) serum aspartate aminotransferase [SGOT] / alanine
   aminotransferase (ALT) serum glutamic pyruvic transaminase [SGPT] =< 3 x institutional
   ULN

   - Patients must be willing to provide tumor tissue from the diagnostic core biopsy. If
   inadequate tumor tissue is available, patients are still eligible to participate in
   the trial

   - Patients with a prior or concurrent malignancy whose natural history or treatment does
   not have the potential to interfere with the safety or efficacy assessment of the
   investigational regimen are eligible for this trial

   - Patients with known history or current symptoms of cardiac disease, or history of
   treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac
   function using the New York Heart Association Functional Classification. To be
   eligible for this trial, patients should be class 2B or better

   - Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral
   therapy with undetectable viral load within 6 months prior to registration are
   eligible for this trial

Exclusion Criteria:

   - No stage IV (metastatic) breast cancer

   - No history of any prior (ipsi- or contralateral) invasive breast cancer. Prior DCIS is
   allowed

   - No evidence of recurrent disease following preoperative therapy and surgery

   - No known active liver disease, e.g. due to hepatitis B virus (HBV), hepatitis C virus
   (HCV), autoimmune hepatic disorders, or sclerosing cholangitis

   - No history of intolerance, including Grade 3 or 4 infusion reaction or
   hypersensitivity to pembrolizumab or murine proteins or any components of the product

   * Note: Prior immune-related adverse events (irAEs) are allowed if they resolved and
   the patient tolerated subsequent therapy without requiring chronic steroids for the
   irAE

   - No medical conditions that require chronic systemic steroids (>10 mg prednisone daily
   or equivalent) or any other form of immunosuppressive medications and has required
   such therapy in the last two years. Replacement therapy (e.g. thyroxine, insulin, or
   physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency,
   etc.) is not considered a form of systemic therapy

   - Patients who are unable or unwilling to comply with the requirements of the protocol
   per investigator assessment are not eligible