This Phase II, single-arm study is designed to evaluate the effect of atezolizumab treatment in participants with locally advanced or metastatic urothelial bladder cancer. Participants will be enrolled into 1 of 2 cohorts. Cohort 1 (reported here) will consist of participants who are treatment-naïve and ineligible for cisplatin-containing chemotherapy. Cohort 2 will contain participants who have progressed during or following a prior platinum-based chemotherapy regimen. The results of the second cohort are reported separately (NCT02108652). Participants in both cohorts will be given a 1200 milligrams (mg) intravenous (IV) dose of atezolizumab on Day 1 of 21-day cycles. Treatment of participants in Cohort 1 will continue until disease progression per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST v1.1) or unmanageable toxicity. Treatment of participants in Cohort 2 will continue until loss of clinical benefit or unmanageable toxicity.