Inclusion Criteria: - Diagnosed with metastatic castration-resistant prostate cancer (mCRPC) - A confirmed homologous recombination repair (HRR) gene alteration as identified by a genomic assay through use of a validated test - Eastern Cooperative Oncology Group Performance Score (ECOG PS) Grade of 0 or 1 - Willing to receive concomitant prednisone/ prednisolone 10 milligrams (mg) (not provided by Janssen) daily while receiving niraparib/abiraterone acetate (Nira/AA) combination treatment - Willing to continue treatment with gonadotropin-releasing hormone (GnRH) analogue if not surgically castrated

Exclusion Criteria: - Prior treatment with a Poly ADP-ribose Polymerase (PARP) inhibitor - Prior systemic therapy for mCRPC (that is, taxane-based chemotherapy, androgen signaling inhibitors). Ongoing androgen deprivation therapy (ADT), prior systemic treatment (example, apalutamide, enzalutamide, darolutamide, docetaxel) for earlier disease settings (example, hormone sensitive disease, non-metastatic castration-resistant disease); and treatment recently initiated with abiraterone acetate for mCRPC less than or equal to (