BACKGROUND AND AIMS: Data on upadacitinib therapy in children with ulcerative colitis (UC) or unclassified inflammatory bowel disease (IBD-U) are scarce. We aimed to evaluate the effectiveness and safety of upadacitinib as an induction therapy in pediatric UC or IBD-U. METHODS: In this multicenter retrospective study, children treated with upadacitinib for induction of remission of active UC or IBD-U from 30 centers worldwide were enrolled. Demographic, clinical and laboratory data as well as adverse events (AEs) were recorded at week 8 post induction. RESULTS: One hundred children were included (90 UC and 10 IBD-U, median age 15.6 [interquartile range 13.3-17.1] years). Ninety-eight were previously treated with biologic therapies, and 76 were treated with ≥2 biologics. At the end of the 8-week induction period, clinical response, clinical remission, and corticosteroid-free clinical remission (CFR) were observed in 84%, 62%, and 56% of the children, respectively. Normal C-reactive protein and fecal calprotectin (FC) <150 mcg/g were achieved in 75% and 50%, respectively. Combined CFR and FC remission was observed in 18/46 (39%) children with available data at 8 weeks. AEs were recorded in 37 children, including one serious AE of an appendiceal neuroendocrine tumor. The most frequent AEs were hyperlipidemia (n=13), acne (n=12), and infections (n=10, five of whom with herpes viruses). CONCLUSION: Upadacitinib is an effective induction therapy for refractory pediatric UC and IBD-U. Efficacy should be weighed against the potential risks of AEs.