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 (mg) by subcutaneous (SC) injection once every 4 weeks (Q4W) + placebo (intravenous [IV] infusion at Weeks 0, 2, and 6, then once every 8 weeks [Q8W]) or infliximab 5 milligrams/kilogram (mg/kg) IV at Weeks 0, 2, and 6, then Q8W) + placebo (SC Q4W). Time on treatment is 54 weeks.