Ulcerative colitis (UC) is an idiopathic, chronic, inflammatory disease affecting the colon. Participants with UC have mucosal inflammation starting in the rectum that can extend continuously to proximal segments of the colon. This study will assess how safe and effective upadacitinib is in treating adult participants with moderate to severe ulcerative colitis (UC). Adverse events and change in disease activity will be assessed. Upadacitinib is a drug approved for the treatment of Ulcerative colitis (UC). All study participants will receive upadacitinib as prescribed by their study doctor in accordance with approved local label. Approximately 1000 adult participants will be enrolled worldwide. Participants will receive upadacitinib as prescribed by their physician according to their routine clinical practice and local label. Participants will be followed for up to 3 years. There is expected to be no additional burden for participants in this trial. Participants will attend regular visits during the study at a hospital or clinic according to their routine clinical practice.