This is a randomized withdrawal, double blind, placebo controlled study of pediatric subjects (2 to <18 years of age) with JIA. The primary objective is to compare the efficacy of tofacitinib versus placebo for the treatment of signs and symptoms of JIA at Week 26 of the double blind phase as measured by the percentage of subjects with disease flare (according to PRCSG/PRINTO Disease Flare criteria) after Week 18 of the open label run in phase.All eligible subjects enrolled in the study will initially receive open label tofacitinib for 18 weeks (run in phase). At the end of the 18 week run in phase, only subjects who achieve at least a JIA ACR 30 response will be randomized to the 26 week double blind, placebo controlled phase. Subjects who do not achieve a JIA ACR 30 response at this time point will be discontinued from the study. In addition, subjects who experience a single episode of disease flare at any time during the study (including the open label run in and double blind phase) will also be discontinued from the study. All subjects participating in this study, including those discontinued from the study, will have the option, if eligible (based on inclusion and exclusion criteria), of enrolling in the tofacitinib JIA long term extension study (A3921145).