Background: Phase 3 trials have assessed efficacy of abrocitinib versus placebo in moderate-to-severe atopic dermatitis, a common immunoinflammatory skin disease. This study assessed the efficacy and safety of abrocitinib versus dupilumab. Methods: This randomised, double-blind, double-dummy, active-controlled, parallel-treatment, phase 3 trial enrolled adults with moderate-to-severe atopic dermatitis who requir=ed systemic therapy or had inadequate response to topical medications. Participants were enrolled from 151 sites, located in Australia, Bulgaria, Canada, Chile, Finland, Germany, Hungary, Italy, Latvia, Poland, Slovakia, South Korea, Spain, Taiwan, and the USA. These participants were then randomly assigned (1:1) with block randomisation to receive oral abrocitinib (200 mg per day) or subcutaneous dupilumab (300 mg every 2 weeks) for 26 weeks. Participants were required to apply topical corticosteroids (medium or low potency), topical calcineurin inhibitors, or a topical phosphodiesterase 4 inhibitor to active lesion areas. Primary endpoints were response based on achieving a 4 point or higher improvement in Peak Pruritus Numerical Rating Scale (PP-NRS4) at week 2 and a 90% or better improvement in Eczema Area and Severity Index (EASI-90) at week 4. Family-wise type 1 error was controlled via a sequential multiple-testing procedure (two sided, α=0·05). Randomly assigned participants who received at least one dose of study intervention were included in the efficacy and safety analysis sets. This trial was completed on July 13, 2021 (NCT04345367). Findings: Between June 11, 2020, and Dec 16, 2020, 940 patients were screened and 727 were enrolled (362 in the abrocitinib group and 365 in the dupilumab group). Compared with dupilumab, a larger proportion of patients treated with abrocitinib reached the primary outcomes, PP-NRS4 at week 2 (172 [48%] of 357, 95% CI 43·0–53·4 vs 93 [26%] of 364, 21·1–30·0; difference 22·6%, 15·8–29·5; p<0·0001), and EASI-90 at week 4 (101 [29%] of 354, 23·8–33·2 vs 53 [15%] of 364, 10·9–18·2; difference 14·1%, 8·2–20·0; p<0·0001). Treatment-emergent adverse events were reported by 268 (74%) of 362 patients treated with abrocitinib and by 239 (65%) of 365 patients treated with dupilumab. Two non-treatment-related deaths occurred in the abrocitinib group. Interpretation: Abrocitinib 200 mg per day was more efficacious than dupilumab in adults with moderate-to-severe atopic dermatitis on background topical therapy in inducing early reductions of itch and atopic dermatitis disease signs. Both treatments were well tolerated over 26 weeks. Funding: Pfizer.