Psychological inflexibility has been associated with several mental health indicators. The Avoidance and Fusion Questionnaire for Youth (AFQ-Y) has been the most used measure to assess psychological inflexibility in adolescents, having been adapted to several different languages, including Portuguese. However, the short 8-item version of this scale has been thoroughly studied and considered a more advantageous version, but it has not yet been validated for the Portuguese population. Therefore, the current study aims to contribute to the establishment of the validity and reliability of the shorter version of AFQ-Y by validating the AFQ-Y8 in a Portuguese population of adolescents and exploring its psychometric properties. The present study was conducted in two different samples of adolescent boys and girls, with an age range between 12 and 18. Confirmatory factor analyzes (CFA) were used to assess the scales' structure. Reliabilities and other validities were also analyzed. CFA results confirmed the plausibility of the unidimensional structure of the AFQ-Y8, showing adequate fit indices. The AFQ-Y8 also presented an adequate internal consistency and test-retest reliability. Correlation results demonstrated that the AFQ-Y8 was positively associated with measures of anxiety and depression and negatively linked with a measure of mindfulness. These correlations were also significant when the effect of mindfulness skills was accounted for. Overall, the AFQ-Y8 was demonstrated as a valid and reliable measure, and its validation contributes not only to research but also to clinical practice. The use of the AFQ-Y8 in clinical and educational settings may contribute to young-related early detection and initial referral to adequate treatment.