The Acceptance and Commitment Therapy (ACT) literature suggests that psychological inflexibility (PI) is associated with an elevated risk of the development and exacerbation of mental health problems. PI is likely to play a role in the development and maintenance of psychological problems among adolescents, but there is only one valid measure of PI for children and adolescents: Avoidance and Fusion Questionnaire for Youth (AFQ-Y). This measure has been translated into other languages and validated across different cultures, but it has not yet been validated using Iranian samples. Thus, the present study is the first to have examined the psychometric properties of the Persian adaptation of the AFQ-Y and its shorter version (AFQ-Y8). A total of 400 students, 12–18-year-old, were recruited from schools in Mashhad. A multimethod approach was adopted, and the following psychometric properties of the AFQ-Y were examined: internal consistency, test-retest reliability, dimensionality, and concurrent, convergent, and discriminant validity. The participants completed the AFQ-Y, Acceptance and Action Questionnaire-II (AAQ-II), Cognitive Fusion Questionnaire (CFQ), and Depression Anxiety Stress Scales (DASS-21). The fit indices indicated that the one-factor model of the short form (AFQ-Y8) was a good fit for the data and the one-factor model of the long form had to be modified in order to achieve a good fit. The moderate path coefficients of scores on AFQ-Y (short and long forms) on depression, anxiety, and stress were indicative of adequate concurrent validity. Further, both the short and long forms of the AFQ-Y were significantly correlated with the AAQ-II and CFQ; thus, both forms demonstrated satisfactory convergent validity. The Fornell-Larcker criterion was fulfilled; further, the items loaded onto their intended factors more strongly than they loaded onto other factors (i.e., cross-loadings). With regard to reliability, both forms were found to have high internal consist
