This is a multicenter, Phase III, randomized, double-blind, double-dummy, parallel-group study to evaluate the safety, efficacy, and tolerability of etrolizumab compared with infliximab in treating participants with moderate to severe ulcerative colitis (UC) who are naive to tumor necrosis factor (TNF) inhibitors. Participants will be randomized in a 1:1 ratio to receive either etrolizumab 105 (milligrams) by subcutaneous (SC) injection [SC] every 4 weeks [Q4W]) + placebo (intravenous [IV] infusion at Weeks 0, 2, and 6, then every 8 weeks) or infliximab 5 milligrams/kilogram (mg/kg) IV at Weeks 0, 2, and 6, then every 8 weeks) + placebo (SC Q4W). Time on treatment is 54 weeks.