Vitiligo is a chronic autoimmune disorder leading to skin depigmentation and reduced quality of life (QOL). Patients with extensive and very active disease are the most difficult to treat. To assess the efficacy and adverse events of baricitinib combined with narrowband UV-B in adults with severe, active, nonsegmental vitiligo. This academic, multicenter, double-blind, noncomparative randomized clinical trial was conducted at 4 dermatology departments between July 2021 and April 2023 and included adult patients with extensive and active nonsegmental vitiligo. The study was designed to evaluate the effect of baricitinib plus narrowband UV-B based solely on the results from this experimental group. The placebo group was used as a calibration group. Data were analyzed from August to November 2023. Participants were randomized 3:1 to baricitinib, 4 mg per day, or placebo for 36 weeks alone for the first 12 weeks and then in combination with narrowband UV-B twice a week from weeks 12 to 36. The primary outcome was mean percentage change in total Vitiligo Area Scoring Index (VASI) score from baseline to week 36 (baricitinib group). The prespecified aim of the study was to show that the reduction in the baricitinib plus narrowband UV-B was significantly greater than 42.9%, a repigmented surface threshold previously observed in patients treated with narrowband UV-B alone. Adverse events and secondary outcomes of change in disease activity and QOL were assessed. Post hoc analyses were additionally performed. Of 49 included patients, 35 (71%) were female, and the median (IQR) age was 49.9 (38.4-59.8) years. A total of 37 patients were randomized to the baricitinib group and 12 to the placebo group. The mean change in total VASI at week 36 was -44.8% (95% CI, -58.4% to -31.3%) for the baricitinib group and -9.2% (95% CI, -27.7% to 24.7%) for the placebo group. This was not significantly greater than the sufficient repigmented surface threshold of 42.9%. Post hoc analyses showed a significant difference at week 36 for total VASI score in the baricitinib plus narrowband UV-B group compared with placebo plus narrowband UV-B (-44.8% vs -9.2%, respectively; P = .02). There was a greater improvement in disease activity and QOL in the baricitinib group vs placebo group and no significant difference in the number of adverse events. This proof-of-concept randomized clinical trial confirmed the efficacy of baricitinib combined with narrowband UV-B in the treatment of patients with extensive and active vitiligo. ClinicalTrials.gov Identifier: NCT04822584.