Inclusion Criteria:

          -  Has newly diagnosed, locally advanced, centrally confirmed TNBC, as defined by the
             most recent American Society of Clinical Oncology (ASCO)/College of American
             Pathologists (CAP) guidelines.

          -  Has previously untreated locally advanced non-metastatic (M0) TNBC defined as the
             following combined primary tumor (T) and regional lymph node (N) staging per current
             American Joint Committee of Cancer (AJCC) staging criteria for breast cancer as
             assessed by the investigator based on radiological and/or clinical assessment:

               -  T1c, N1-N2

               -  T2, N0-N2

               -  T3, N0-N2

               -  T4a-d, N0-N2

          -  Provides a core needle biopsy consisting of at least 2 separate tumor cores from the
             primary tumor at screening to the central laboratory.

          -  Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 performed
             within 10 days of treatment initiation.

          -  Demonstrates adequate organ function.

          -  Males and female participants of childbearing potential must be willing to use an
             adequate method of contraception for the course of the study through 12 months after
             the last dose of study treatment for participants who have received cyclophosphamide,
             and 6 months after the last dose of study treatment for participants who did not.

        Exclusion Criteria:

          -  Has a history of invasive malignancy ≤5 years prior to signing informed consent except
             for adequately treated basal cell or squamous cell skin cancer or in situ cervical
             cancer.

          -  Has received prior chemotherapy, targeted therapy, and radiation therapy within the
             past 12 months.

          -  Has received prior therapy with an anti-programmed cell death protein 1 (anti-PD-1),
             anti-programmed death - ligand 1 (anti-PD-L1), or anti-PD-L2 agent or with an agent
             directed to another co-inhibitory T-cell receptor (e.g., cytotoxic
             T-lymphocyte-associated antigen-4 [CTLA-4], OX-40, CD137 [tumor necrosis factor
             receptor superfamily member 9 (TNFRSF9)]) or has previously participated in a
             pembrolizumab (MK-3475) clinical study.

          -  Is currently participating in or has participated in an interventional clinical study
             with an investigational compound or device within 4 weeks of the first dose of
             treatment in this current study.

          -  Has received a live vaccine within 30 days of the first dose of study treatment.

          -  Has an active autoimmune disease that has required systemic treatment in past 2 years
             (i.e., with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs).

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy (i.e.,
             dosing exceeding 10 mg daily of prednisone or equivalent) or any other form of
             immunosuppressive therapy within 7 days prior to the first dose of study treatment.

          -  Has a known history of Human Immunodeficiency Virus (HIV).

          -  Has known active Hepatitis B or Hepatitis C.

          -  Has a history of (non-infectious) pneumonitis that required steroids or current
             pneumonitis.

          -  Has an active infection requiring systemic therapy.

          -  Has significant cardiovascular disease, such as: history of myocardial infarction,
             acute coronary syndrome or coronary angioplasty/stenting/bypass grafting within the
             last 6 months OR congestive heart failure (CHF) New York Heart Association (NYHA)
             Class II-IV or history of CHF NYHA Class III or IV.

          -  Is pregnant or breastfeeding, or expecting to conceive children within the projected
             duration of the study, starting with the screening visit through 12 months after the
             last dose of study treatment for participants who have received cyclophosphamide, and
             for 6 months after the last dose of study treatment for participants who have not.

          -  Has a known hypersensitivity to the components of the study treatment or its analogs.

          -  Has a known history of active tuberculosis (TB, Bacillus Tuberculosis).