The purpose of this study is to evaluate the efficacy and safety of tislelizumab plus chemotherapy vs chemotherapy alone as perioperative treatment in participants who have triple negative HER2 negative breast cancer.

After a screening phase of approximately 28 days, each participant will receive neoadjuvant study treatment (Tislelizumab + Chemotherapy OR Chemotherapy) based on the randomization schedule for approximately 24 weeks (8 cycles). Each participant will then undergo definitive surgery 3-6 weeks after conclusion of the last cycle of the neoadjuvant study treatment.

After definitive surgery, each participant will receive adjuvant study treatment (routine adjuvant treatment +/- Tislelizumab) for approximately 42 weeks (14 cycles).

Following adjuvant study treatment, each participant will be monitored for safety, survival and disease recurrence.

Inclusion Criteria:

• Pathologically proven diagnosis of invasive breast cancer, cT1-T3, cN0-N3, cM0, HR+ (ER+ and/or PR+) HER2 negative or HR- (ER- and PR-) HER2 negative/triple negative breast cancer.
• Provides a core needle biopsy consisting of at least 2 separate tumor cores from the primary tumor at screening to the central laboratory.
• Immune active subtype revealed by multiplexed
• Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 performed within 10 days of treatment initiation.
• Demonstrates adequate organ function.

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, or radiation therapy for breast cancer.
• 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. CTLA-4, OX-40, CD137)
• 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 known history of active tuberculosis.
• 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.
• Pregnant or lactating women are ineligible.