Efficacy and Safety Comparison of Niraparib to Placebo in Participants With Human Epidermal Growth Factor 2 Negative (HER2-) Breast Cancer Susceptibility Gene Mutation (BRCAmut) or Triple-Negative Breast Cancer (TNBC) With Molecular Disease

Inclusion Criteria:

   - Stage I to III breast cancer with surgical resection of the primary tumor that is
   confirmed to be either: TNBC, irrespective of BRCA status or HR+/HER2- breast cancer
   with a known and documented deleterious or suspected deleterious tBRCA mutation.

   - Estrogen receptor (ER) and/or progesterone receptor (PgR) negativity is defined as
   immunohistochemistry (IHC) nuclear staining less than (<) 1 percentage (%), or by
   Allred scoring system where TNBC is defined to be 0 out of 8 or 2 out of 8, or
   staining in <1 % of cancer cells.

   - Completed prior standard therapy for curative intent.

   - Participants with HR+ breast cancer must be on a stable regimen of endocrine therapy.

   - Detectable ctDNA as measured by central testing.

   - An archival tumor tissue specimen of the primary tumor sufficient in quality and
   quantity for ctDNA assay design and tBRCA and Homologous recombination deficiency
   (HRD) testing is required.

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

Exclusion Criteria:

   - Prior treatment with a Poly Adenosine-diphosphate Ribose Polymerase (PARP) inhibitor.

   - Current treatment with a Cyclin-dependent kinase (CDK)4/6 inhibitor or endocrine
   therapy other than anastrozole, letrozole, exemestane, and tamoxifen with or without
   ovarian suppression.

   - Participants have any sign of metastasis or local recurrence after comprehensive
   assessment conducted per protocol.

   - Participants have shown no definitive response to preoperative chemotherapy by
   pathologic, radiographic or clinical evaluation, in cases where preoperative
   chemotherapy was administered.

   - Participants have inadequately treated or controlled hypertension.

   - Participants have received live vaccine within 30 days of planned start of study
   randomization.

   - Participants have a second primary malignancy.

   - Exceptions are the following: (a) Adequately treated non-melanoma skin cancer,
   curatively treated in situ cancer of the cervix, Ductal carcinoma in situ (DCIS) of
   the breast, Stage I Grade 1 endometrial carcinoma. (b) Other solid tumors and
   lymphomas (without bone marrow involvement) diagnosed >=5 years prior to randomization
   and treated with no evidence of disease recurrence and for whom no more than 1 line of
   chemotherapy was applied.

   - Participant is pregnant, breastfeeding, or expecting to conceive children while
   receiving study treatment and/or for up to 180 days after the last dose of study
   treatment (except France).

   - Participant is immunocompromised. Participants with splenectomy are allowed.
   Participants with known human immunodeficiency virus (HIV) are allowed if they meet
   protocol-defined criteria.

   - Participants have a known history of myelodysplastic syndrome (MDS) or acute myeloid
   leukemia (AML).