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Background: Research has demonstrated the effectiveness of a mobile app-delivered Acceptance and Commitment Therapy (IACT) programme for individuals with posttraumatic stress disorder (PTSD) in their PTSD symptoms. However, there are still few studies on its mediating mechanism.Objective: This secondary analysis aimed to examine whether six components of psychological flexibility - acceptance, present-moment awareness, self-as-context, cognitive defusion, valued living, and committed action - mediate the effects of an IACT programme for individuals with PTSD.Method: Data were obtained from a randomized controlled trial in which participants were assigned to an IACT condition (n = 78) or a waitlist control (WL; n = 67). Using a latent growth curve modelling approach with bias-corrected bootstrapped confidence intervals, changes in PTSD symptoms and psychological flexibility components were assessed at baseline (T1) and following each of six intervention modules (T2-T7).Results: Relative to WL, the IACT group showed significantly greater improvements in both PTSD symptoms and all six flexibility components. Mediation analyses revealed that four processes - acceptance, self-as-context, cognitive defusion, and committed action - fully mediated the intervention's effects on PTSD outcomes, whereas valued living and present-moment awareness served as partial mediators.Conclusions: These findings suggest that enhancing these components of psychological flexibility may be crucial for effective IACT treatment of PTSD.