The purpose of this study is to learn about the effects of the study drug tofacitinib for moderately to severely active ulcerative colitis (UC) in pediatric patients. Tofacitinib is a drug that affects the immune system. The immune system normally fights infection, but in diseases like UC, the immune system may be overactive or act incorrectly, leading to a chronic inflammation in the digestive tract (colon). Tofacitinib may be able to control disease activity, improve symptoms and keep up normal, every-day functions and activities.

Tofacitinib is also known by the trade name Xeljanz and is approved by the U.S. Food and Drug Administration (FDA) for moderately to severely active UC in adults. However, the use of tofacitinib in this research study is investigational because it is not currently approved to treat pediatric UC.

Participants will receive tofacitinib 5 mg twice daily or 10 mg twice daily during all 3 phases of the study: induction, maintenance and extension. If participants respond to tofacitinib in induction after 8 or 16 weeks, they will be eligible to enter the maintenance phase of the study. After 44 weeks in the maintenance phase, participants will enter the extension phase of the study, which will last another 2 years.

Total study participants will last about 3 and a half years. Participants will need to visit the study site about 23 times during the study. These visits will occur about every 1 to 3 months. The final follow-up visit will be conducted by telephone.