A Study of the Efficacy and Safety of Upadacitinib (ABT-494) in Participants With Moderately to Severely Active Ulcerative Colitis

\- Male or female participants ≥ 16 and ≤ 75 years of age at Baseline Note: Adolescent participants at the age of 16 or 17 years old will be eligible to participate if approved by the country or regulatory/health authorities. Note: Adolescent participants at the age of 16 or 17 years old must weigh ≥ 40 kilograms and meet the definition of Tanner Stage 5 at the Screening Visit. * Diagnosis of Ulcerative Colitis (UC) for 90 days or greater prior to Baseline, confirmed by colonoscopy during the Screening Period, with exclusion of current infection, colonic dysplasia and/or malignancy. Appropriate documentation of biopsy results consistent with the diagnosis of UC, in the assessment of the Investigator, must be available. * Active UC with an Adapted Mayo score of 5 to 9 points and endoscopic subscore of 2 to 3. * Demonstrated an inadequate response, loss of response, or intolerance to at lease one of the following treatments including, oral aminosalicylates, corticosteroids, immunosuppressants, and/or biologic therapies. Note: Participants who have had inadequate response, loss of response to conventional therapy but have not failed biologic therapy (Non-bio-IR) and have received a prior biologic for up to 1 year may be enrolled, however they must have discontinued the biologic for reasons other than inadequate response or intolerance (e.g., change of insurance, well controlled disease), and must meet criteria for inadequate response, loss of response, or intolerance as defined above. * Female Participants of childbearing potential must have a negative serum pregnancy test at the Screening Visit and a negative urine pregnancy test at the Baseline Visit. * If female, participant must meet the contraception recommendation criteria.

* Participant with current diagnosis of Crohn's disease (CD) or diagnosis of indeterminate colitis (IC). * Current diagnosis of fulminant colitis and/or toxic megacolon. * Participant with disease limited to the rectum (ulcerative proctitis) during the Screening endoscopy. * Received cyclosporine, tacrolimus, mycophenolate mofetil, or thalidomide within 30 days prior to Baseline. * Participant who received azathioprine or 6-mercaptopurine (6-MP) within 10 days of Baseline. * Received intravenous corticosteroids within 14 days prior to Screening or during the Screening Period. * Participant with previous exposure to Janus Activated Kinase (JAK) inhibitor (e.g., tofacitinib, baricitinib, filgotinib, upadacitinib). * Screening laboratory and other analyses show any prespecified abnormal hematologic results.