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

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