This is a study to evaluate pharmacokinetics, safety and tolerability of upadacitinib in pediatric participants with polyarticular course juvenile idiopathic arthritis. This study consists of three parts: Part 1 is multiple-cohort study that consists of two sequential multiple dose groups. Participants benefiting from the study drug with no ongoing adverse events of special interest or serious adverse events will have option to enroll in Part 2. Part 2 is open-label, long term extension study to evaluate safety and tolerability. Part 3 is an additional safety cohort to evaluate long-term safety and tolerability.

Inclusion Criteria:

• Participant have total body weight of 10 kg or higher at the time of screening.
• Participant diagnosed with pcJIA (rheumatoid factor-positive or rheumatoid factor-negative polyarticular JIA, extended oligoarticular JIA, or systemic JIA with active arthritis and without active systemic features) with a history of arthritis affecting at least 5 joints within the first 6 months of disease (for extended oligoarticular JIA: <=4 joints within first 6 months of disease and >4 joints thereafter).
• Participant have 5 or more active joints at the time of screening, defined as the presence of swollen joints (not due to deformity) or, in the absence of swelling, joints with the limitation of movement (LOM) plus pain on motion and/or tenderness with palpitation, with LOM present in at least three of the active joints.
• If receiving methotrexate (MTX), have been taking MTX for at least 12 weeks immediately before and including Study Day 1 on a stable dose of <=20 mg/m2 for at least 8 weeks before and including Study Day 1; in addition, participants should take either folic acid or folinic acid according to local standard of care.
• If on oral glucocorticosteroids, must have been taking oral glucocorticosteroids at a stable dose (no greater than 10 mg/day or 0.2 mg/kg/day, whatever is lower) for at least 1 week before and including Study Day 1.

Exclusion Criteria:

• Participant with diagnosis of enthesitis-related arthritis (ERA) or juvenile psoriatic arthritis (JPSA).
• Participant have prior exposure to JAK inhibitor.