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).