A Study of Atezolizumab (Anti-PD-L1 Antibody) in Combination With Enzalutamide in Participants With Metastatic Castration-Resistant Prostrate Cancer (mCRPC) After Failure of an Androgen Synthesis Inhibitor And Failure of, Ineligibility For, or Refusal of a Taxane Regimen

Inclusion Criteria

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Life expectancy greater than or equal to (>/=) 3 months
• Histologically confirmed adenocarcinoma of the prostate
• Known castrate-resistant disease with serum testosterone level less than or equal to (</=) 50 nanograms per deciliter (ng/dL) with prior surgical castration or ongoing androgen deprivation for the duration of the study
• Progressive disease prior to screening by PSA or imaging per PCWG3 criteria during or following the direct prior line of therapy in the setting of medical or surgical castration
• One prior regimen/line of a taxane-containing regimen for mCRPC or refusal or ineligibility of a taxane-containing regimen
• Progression on a prior regimen/line of an androgen synthesis inhibitor for prostate cancer
• Availability of a representative tumor specimen from a site not previously irradiated that is suitable for determination of programmed death-ligand 1 (PD-L1) status via central testing
• Adequate hematologic and end organ function

Exclusion Criteria

• Prior treatment with enzalutamide or any other newer hormonal androgen receptor inhibitor (e.g., apalutamide, ODM-201)
• Treatment with any approved anti-cancer therapy, including chemotherapy, immunotherapy, radiopharmaceutical or hormonal therapy (with the exception of abiraterone), within 4 weeks prior to initiation of study treatment
• Treatment with abiraterone within 2 weeks prior to study treatment
• Structurally unstable bone lesions suggesting impending fracture
• Known or suspected brain metastasis or active leptomeningeal disease
• Major surgical procedure other than for diagnosis within 4 weeks prior to initiation of study treatment or anticipation of need for a major surgical procedure during the course of the study
• Active or history of autoimmune disease or immune deficiency
• Prior allogeneic stem cell or solid organ transplantation
• History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis obliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan
• Positive human immunodeficiency virus (HIV) test, active tuberculosis, active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection
• Prior treatment with cluster of differentiation (CD)137 agonists or immune checkpoint blockade therapies, including anti Cytotoxic T Lymphocyte-Associated 4 (CTLA4), anti-programmed death 1 (PD-1), and anti-PD-L1 therapeutic antibodies
• Treatment with systemic immunostimulatory agents within 4 weeks or five half-lives of the drug, whichever is shorter, prior to initiation of study treatment
• Treatment with systemic immunosuppressive medication within 2 weeks prior to initiation of study
• History of seizure or any condition that may predispose to seizure within 12 months prior to study treatment, including history of unexplained loss of consciousness or transient ischemic attack