Ulcerative colitis (UC) is a type of inflammatory bowel disease that causes inflammation and bleeding from the lining of the rectum and colon (large intestine).This study will evaluate how safe and effective risankizumab is compared to vedolizumab in treating adult participants with moderate to severe UC who are naive to targeted therapies (TaTs). Risankizumab and vedolizumab are approved medications for moderate to severe UC in multiple countries. Participants who meet the eligibility criteria will be randomized in a 1:1 ratio to receive open label risankizumab or vedolizumab. Approximately 530 adult participants with moderate to severe UC who are naïve to targeted therapies (TaTs) will be enrolled at 285 sites worldwide. For participants randomized to risankizumab, drug will be administered intravenous(IV) during the induction period followed by subcutaneous injection during the maintenance period. Participants randomized to vedolizumab will receive drug IV throughout the study. The duration of the study is approximately 69 weeks for participants randomized to risankizumab and 71 weeks for participants randomized to vedolizumab. This includes up to a 35-day screening period followed by a treatment period of 44 weeks for risankizumab and 46 weeks for vedolizumab. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular outpatient visits during the study. The effect and safety of the treatment will be checked by medical assessments, evaluation of side effects and completing questionnaires.