Abrocitinib, an oral, once-daily Janus kinase 1 selective inhibitor, was effective and well tolerated in a phase 3 monotherapy trial of patients with moderate-to-severe atopic dermatitis (AD). To investigate the efficacy and safety of abrocitinib in adolescents and adults with moderate-to-severe AD in an identically designed trial. This phase 3, double-blinded, placebo-controlled, parallel-group randomized clinical trial included patients 12 years or older with a clinical diagnosis of moderate-to-severe AD for at least 1 year and inadequate response to topical medications given for at least 4 weeks within 6 months. Patients were enrolled from 115 centers in Australia, Bulgaria, Canada, China, Czechia, Germany, Hungary, Japan, South Korea, Latvia, Poland, United Kingdom, and the United States from June 29, 2018, to August 13, 2019. Data were analyzed from September 13 to October 25, 2019. Patients were randomly assigned (2:2:1) to receive once-daily oral abrocitinib in 200- or 100-mg doses or placebo for 12 weeks. The coprimary end points were the proportion of patients achieving Investigator Global Assessment (IGA) response (ie, clear  or almost clear , with improvement of ≥2 grades) and the proportion of patients achieving at least 75% improvement in Eczema Area and Severity Index score (EASI-75) at week 12. Key secondary end points included the proportion of patients achieving a Peak Pruritus Numerical Rating Scale (PP-NRS) response (ie, improvement of ≥4 points) at week 12. Other secondary end points included the proportion of patients achieving at least 90% improvement in EASI score (EASI-90). Safety was assessed via adverse events and laboratory monitoring. A total of 391 patients (229 male [58.6%]; mean [SD] age, 35.1 [15.1] years) were included in the analysis; of these, 155 received abrocitinib, 200 mg/d; 158, abrocitinib, 100 mg/d; and 78, placebo. Among patients with available data at week 12, greater proportions of patients in the 200- and 100-mg abrocitinib groups vs the placebo group achieved IGA (59 of 155 [38.1%] and 44 of 155 [28.4%] vs 7 of 77 [9.1%]; P < .001) and EASI-75 (94 of 154 [61.0%] and 69 of 155 [44.5%] vs 8 of 77 [10.4%]; P < .001), greater estimated proportions achieved PP-NRS (55.3% [95% CI, 47.2%-63.5%] and 45.2% [95% CI, 37.1%-53.3%] vs 11.5% [95% CI, 4.1%-19.0%]; P < .001), and/or greater proportions achieved EASI-90 (58 of 154 [37.7%] and 37 of 155 [23.9%] vs 3 of 77 [3.9%]) responses. Adverse events were reported for 102 patients (65.8%) in the 200-mg group, 99 (62.7%) in the 100-mg group, and 42 (53.8%) in the placebo group; serious adverse events were reported for 2 patients (1.3%) in the 200-mg group, 5 (3.2%) in the 100-mg group, and 1 (1.3%) in the placebo group. Decreases in platelet count (2 [1.3%]) and laboratory values indicating thrombocytopenia (5 [3.2%]) were reported in the 200-mg group. Monotherapy with once-daily oral abrocitinib was effective and well tolerated in adolescents and adults with moderate-to-severe AD. ClinicalTrials.gov Identifier: NCT03575871.