The Avoidance and Fusion Questionnaire for Youth-Short Form (AFQ-Y8) was designed to measure psychological inflexibility in children and adolescents. However, it has not yet been validated to the Spanish population. The present study examines the factorial structure, internal consistency, convergent and incremental validity, and measurement invariance between children and adolescents, male and female, and male and female across age of the AFQ-Y8 with 982 students from Spain (children = 459, adolescents = 523). The results confirmed, in both children and adolescents, a unifactorial structure of general psychological inflexibility. AFQ-Y8 was positively related to negative psychological health outcomes, and negatively related to positive psychological health outcomes and acceptance-related skills, such as mindfulness. In addition, psychological inflexibility showed incremental validity in the explanation of negative psychological health over mindfulness. Measurement invariance across age and gender groups was supported, suggesting that the scores in the AFQ-Y8 are comparable between children and adolescents, and between males and females. In conclusion, results show that the Spanish version of the AFQ-Y8 is a reliable and valid measure of psychological inflexibility in children and adolescents. (PsycInfo Database Record (c) 2020 APA, all rights reserved).