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Early access to evidence-based help is crucial for adolescents with anxiety disorders. Internet-delivered acceptance and commitment therapy (iACT) may offer adolescents increased access to care and more flexibility in engaging with treatment when and how they prefer. Process-based therapies, such as ACT, focus on theoretically derived and empirically tested key mechanisms in treatment that enable change. This study aimed to investigate the effectiveness of iACT for adolescents with anxiety disorders. The study also assessed the relationship between psychological flexibility and treatment outcomes and the relationship between participating adolescents’ and therapists’ perceived alliance and treatment outcomes. This was a randomized controlled trial comparing a 10-week intervention group with a wait-list control group. The 52 participants, aged 15 to 19, were recruited from all over Sweden. The treatment was effective in increasing quality of life and psychological flexibility, with moderate between-group effect sizes based on observed values. Changes in psychological flexibility was associated with changes in anxiety symptoms. The results further showed a statistically significant between-group difference in post-treatment diagnoses. No significant time per group interaction was found for anxiety symptoms, as both groups improved. Working alliance was rated as high by both participating adolescents and therapists but showed no significant relationship with treatment outcomes. Participants found the treatment an acceptable intervention. This study shows promising results for iACT in treating adolescents with anxiety disorders. The results suggest the model of psychological flexibility as an important process of change in treatment outcomes. Future research should validate these findings in larger samples and clinical contexts.