Background: Physical activity (PA) is considered an important factor affecting academic achievement (AA). Different intensities of PA affect mental engagement (ME), which, in turn, affects AA. However, the role of ME in the relationship between PA and AA remains unclear. Objective: This study aimed to examine the mediating and/or moderating role of ME (i.e. cognitive flexibility [COGFLEX], metacognition [META] and competitiveness achievement motivation [COMPETE]) in the relationship between PA (both moderate- and vigorous-intensity activities) and AA. Method: Structural equation modelling was used to build a mediated moderation model. A total of 68,144 students from eight economies who participated in the 2018 Programme for International Student Assessment were included in the study. Results: Moderate-intensity physical activity (MPA) was significantly positively correlated and vigorous-intensity physical activity (VPA) was significantly negatively correlated with AA in adolescents. COGFLEX, META and COMPETE were found to play a significant mediating role in the relationship between both types of PA (MPA and VPA) and AA. COGFLEX and COMPETE were found to moderate the relationship between VPA and AA. Conclusion: ME plays a mediated moderation role in the relationship between the intensity of PA and AA. The theoretical and practical implications of these findings are discussed.