A Study to Assess the Efficacy and Safety of Induction Therapy With Afimkibart (Also Known as RO7790121) in Participants With Moderately to Severely Active Ulcerative Colitis

This Phase III, multicenter, double-blind, placebo-controlled study will evaluate the efficacy and safety of induction therapy with Afimkibart (RO7790121) compared with placebo in participants with moderately to severely active ulcerative colitis (UC).

Inclusion Criteria: * Confirmed diagnosis of UC * Moderately to severely active UC assessed by mMS * Bodyweight \>= 40 kilogram (kg) * Up to date with colorectal cancer (CRC) screening performed according to local standards * Demonstrated inadequate response, loss of response and/or intolerance to at least one protocol-specified conventional or advanced UC therapy * Males and females of childbearing potential must meet protocol criteria for contraception requirements Exclusion Criteria: * Currently known complications of UC (e.g. fulminant colitis, toxic megacolon) * Current diagnosis of Crohn's disease (CD) or indeterminate colitis, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis * Presence of an ostomy or ileoanal pouch * Current diagnosis or suspicion of primary sclerosing cholangitis * Pregnancy or breastfeeding, or intention of becoming pregnant during the study * Past or current evidence of definite low-grade or high-grade colonic dysplasia or adenomas or neoplasia not completely removed * History of malignancy within 5 years, with the exception of malignancies adequately treated with resection for non-metastatic basal cell or squamous cell cancer or in situ cervical cancer * Evidence of infection with Clostridioides difficile (C. difficile; formerly known as Clostridium difficile), cytomegalovirus (CMV), human immunodeficiency virus (HIV), Hepatitis B (HBV), Hepatitis C (HCV) * Has evidence of active tuberculosis (TB), latent TB not successfully treated (per local guidance) or inadequately treated TB * Has received protocol-specified prohibited medicines, including known exposure to any type of anti-TL1A therapy