This research study is for people who have moderately to severely active Ulcerative Colitis (UC). The study is known as the QUASAR study and consists of 3 distinct study phases; Induction, Maintenance, and Long-Term Extension (LTE) of the Maintenance Study.

The main purpose of the QUASAR study is to see if a drug called "guselkumab" is safe and effective in treating UC. Another purpose is to determine how long guselkumab stays in and acts on the body. Individual who joins the QUASAR study will undergo a Screening process which will last a maximum period of 8 weeks to determine if the individual qualifies to participate, before enrolling into the Induction Study (either 1 or 2) for a maximum of 24 weeks. If UC symptoms improve based on certain criteria, and the participant remains eligible according to other study requirements, then the participant will be able to enter the Maintenance Study for 44 weeks. After completion of the Maintenance Study and if, in the opinion of the investigator, the participant may benefit from continued study drug treatment, the participant will have the opportunity to take part in the LTE of the Maintenance Study for up to an additional 2 years.

Inclusion Criteria:

• Documented diagnosis of ulcerative colitis (UC)
• Moderately to severely active UC, defined by modified Mayo score
• Demonstrated inadequate response or intolerance to medical therapies specified in the protocol
• Screening laboratory test results within the parameters specified in the protocol

Exclusion Criteria:

• Diagnosis of indeterminate colitis, microscopic colitis, ischemic colitis, or Crohn's disease or clinical findings suggestive of Crohn's disease
• UC limited to the rectum only or to less than (<) 20 centimeter (cm) of the colon
• Presence of a stoma
• Presence or history of a fistula
• Receiving prohibited medications and/or treatment