Inclusion Criteria:

1. Body weight of ≥ 40 kg at Baseline Visit for participants between ≥ 12 and < 18 years of age
2. Chronic atopic dermatitis (AD) with onset of symptoms at least 3 years before Baseline Visit and subject meets Hanifin and Rajka criteria.
3. Active moderate to severe AD defined by:

1. Eczema Area and Severity Index (EASI) score ≥ 16 at the Screening and Baseline Visits;
2. Validated Investigator's Global Assessment (vIGA) score ≥ 3 at the Screening and Baseline Visits;
3. ≥ 10% Body surface area (BSA) of AD involvement at the Screening and Baseline Visits;
4. Baseline weekly average of daily Worst Pruritus NRS ≥ 4.
5. Candidate for systemic therapy or have recently required systemic therapy for AD
6. Subject has applied a topical emollient (moisturizer) twice daily for at least 7 days before the Baseline Visit.
7. Documented history of inadequate response to topical corticosteroids (TCS) or topical calcineurin inhibitor (TCI) or documented systemic treatment for AD within 6 months before Baseline Visit

Exclusion Criteria:

1. Prior exposure to any Janus kinase (JAK) inhibitor
2. Unable or unwilling to discontinue current atopic dermatitis treatments prior to the study
3. Requirement of prohibited medications during the study
4. Other active skin diseases or skin infections requiring systemic treatment or would interfere with appropriate assessment of atopic dermatitis lesions
5. Female subject who is pregnant, breastfeeding, or considering pregnancy during the study