The effectiveness of transition programs from paediatric to adult healthcare in adolescents with inflammatory bowel disease is not clear, as prospective studies using validated outcome measures for transition are lacking. This study aimed to develop and validate a quantitative Transition Success Score, and to apply it in a multicenter setting to assess the effectiveness of transitional care. The Top 10 outcome items related to successful transition, identified through an international Delphi study with IBD stakeholders, were integrated into a generic questionnaire, the Transition Success Score. In a prospective, multicenter study, Transition Success Score was scored by adult healthcare providers, young adult patients and caregivers, 9-15 months after transfer of care. In seven Dutch hospitals, 160 patients completed the Transition Success Score. The mean score was 25 (range 17-27), 25.6% of patients achieving maximum score. Hypothesis testing for construct validity revealed significant associations with characteristics related to transitional care, such as knowledge, independence, and quality of life (p <0.005). Structural validation indicated the score was most effective at discerning lower levels of transition success. Internal consistency was acceptable (0.64). High disease burden, exacerbation during or after transfer, and certain personality profiles were associated with lower scores. The Transition Success Score serves as a quantitative tool to evaluate the effectiveness of transitional care interventions and to identify inflammatory bowel disease patients at risk of encountering challenges during the transition to adult healthcare.