A Study of Niraparib in Combination With Abiraterone Acetate and Prednisone Versus Abiraterone Acetate and Prednisone for the Treatment of Participants With Deleterious Germline or Somatic Homologous Recombination Repair (HRR) Gene-MutatedProstatic Neoplasms (mCSPC)

Inclusion criteria:

* Pathological diagnosis of prostate adenocarcinoma
* Must have appropriate deleterious homologous recombination repair (HRR) gene alteration
* Metastatic disease as documented by conventional imaging with computed tomography (CT) or magnetic resonance imaging (MRI) (for soft tissue lesions) or 99mTc bone scan (for bone lesions). Participants with a single bone lesion on Technetium-99m (99mTc) bone scan with no other non-nodal metastatic disease must have confirmation of bone metastasis by CT or MRI. Participants with lymph node-only disease are not eligible
* Androgen deprivation therapy (either medical or surgical castration) must have been started \>=14 days prior to randomization and participants be willing to continue androgen deprivation therapy (ADT) through the treatment phase
* Other allowed prior therapy for metastatic castration-sensitive prostate cancer (mCSPC): (a) maximum of 1 course of radiation and 1 surgical intervention for symptomatic control of prostate cancer (example, uncontrolled pain, impending spinal cord compression or obstructive symptoms). Participants with radiation or surgical interventions to all known sites of metastatic disease will be excluded from trial participation. Radiation must be completed prior to randomization (b) Up to a maximum of 6 months of ADT prior to randomization; (c) Up to a maximum of 45 days of abiraterone acetate + prednisone (AA-P) prior to randomization (d) Up to a maximum of 2 weeks of ketoconazole for prostate cancer prior to randomization

Exclusion criteria:

* Prior treatment with a poly (adenosine diphosphate-ribose) polymerase inhibitor (PARP inhibitor)
* History of adrenal dysfunction
* Long-term use of systemically administered corticosteroids (greater than \[\>\] 5 milligrams \[mg\] of prednisone or the equivalent) during the study is not allowed. Short-term use (\<=4 weeks, including taper) and locally administered steroids (for example, inhaled, topical, ophthalmic, and intra-articular) are allowed, if clinically indicated
* History or current diagnosis of myelodysplastic syndrome (MDS)/ acute myeloid leukemia (AML)