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.
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.
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).