Additional Study Information: The purpose of this study is to compare the effects, good or bad, of RO7790121 versus placebo inpatients with ulcerative colitis. In this study, you will get either RO7790121 or placebo. You will have the opportunity to participate in an open-label extension phase (getting the active drug - RO7790121), if your condition has not improved. RO7790121 is an investigational drug, not approved by United States Food and Drug Administration (U.S. FDA) for the treatment of ulcerative colitis. RO7790121 is an antibody directed against a protein called TL1A. TL1A is a protein found naturally in the body that has a role in inflammation. It has been found that TL1A levels and activity are increased in patients with ulcerative colitis. This increase is thought to lead to the development and worsening of the disease. Earlier research has shown that patients with ulcerative colitis have benefited from treatment with RO7790121. The study will enroll up to 400 participants across global investigational sites. Your participation in this study will be to approximately 70 weeks, depending on how your ulcerative colitis responds to the study drug. Eligibility: Age 18 to 80 years; Moderately to severely active Ulcerative Colitis, who have failed prior conventional therapy (aminosalicylates, corticosteroids and/or immunosuppressants) OR prior advanced therapy, which includes biologics or targeted small molecules, e.g., anti-TNF, anti-IL12/23, anti-integrin, S1P receptor modulators, JAK inhibitors, etc.