Purpose: Adolescents with congenital heart disease transition from childhood to adulthood and transfer from pediatric-oriented to adult-oriented care. High-level empirical evidence on the effectiveness of transitional care is scarce. This study investigated the empowering effect (primary outcome) of a structured person-centered transition program for adolescents with congenital heart disease and studied its effectiveness on transition readiness, patient-reported health, quality of life, health behaviors, disease-related knowledge, and parental outcomes e.g., parental uncertainty, readiness for transition as perceived by the parents (secondary outcomes). Methods: The STEPSTONES-trial comprised a hybrid experimental design whereby a randomized controlled trial was embedded in a longitudinal observational study. The trial was conducted in seven centers in Sweden. Two centers were allocated to the randomized controlled trial-arm, randomizing participants to intervention or control group. The other five centers were intervention-naïve centers and served as contamination check control group. Outcomes were measured at the age of 16 years (baseline), 17 years, and 18.5 years. Results: The change in empowerment from 16 years to 18.5 years differed significantly between the intervention group and control group (mean difference = 3.44; 95% confidence interval = 0.27–6.65; p = .036) in favor of intervention group. For the secondary outcomes, significant differences in change over time were found in parental involvement (p = .008), disease-related knowledge (p = .0002), and satisfaction with physical appearance (p = .039). No differences in primary or secondary outcomes were detected between the control group and contamination check control group, indicating that there was no contamination in the control group. Discussion: The STEPSTONES transition program was effective in increasing patient empowerment, reducing parental involvement, improving satisfaction with physical appearance, and increasing disease-related knowledge. © 2023 Society for Adolescent Health and Medicine