Inclusion Criteria:

• Has a clinical diagnosis of Ulcerative Colitis (UC) at least 3 months before Screening
• Has moderately to severely active UC, defined as a baseline (Week 0) Mayo score of 6 to 12, including a screening endoscopy subscore of the Mayo score ≥ 2 as determined by a central reading of the video endoscopy
• Has received treatment with 1 or more tumour necrosis factor (TNF) antagonists or vedolizumab at a dose approved for the treatment of UC, and has a documented history of failure to respond to or tolerate such treatment
• OR has not had biologic therapy (TNF antagonists or vedolizumab)
• Has received biologic therapy but has not demonstrated a history of failure to respond to, or tolerate, a biologic therapy and has a prior or current UC medication history that includes at least 1 of the following
  • Inadequate response to or failure to tolerate current treatment with oral corticosteroids or immunomodulators (6-mercaptopurine [6-MP] or azathioprine [AZA]) 
  • History of failure to respond to, or tolerate either oral or IV corticosteroids or immunomodulators (6-MP or AZA)
  • History of corticosteroid dependence (an inability to successfully taper corticosteroids without a return of the symptoms of UC)
• Before the first administration of study agent vedolizumab must have been discontinued for at least 4 months and anti-tumor necrosis factors (TNFs) for at least 8 weeks

Exclusion Criteria

• Has severe extensive colitis and is at imminent risk of colectomy
• Has UC limited to the rectum only or to < 20 centimeters (cm) of the colon
• Presence of a stoma or history of a fistula
• History of extensive colonic resection (for example, less than 30 cm of colon remaining) that would prevent adequate evaluation of the effect of study agent on clinical disease activity
• History of colonic mucosal dysplasia.
  • Participants will not be excluded from the study because of a pathology finding of "indefinite dysplasia with reactive atypia"