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 (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