OBJECTIVES: To explore the associations between objectively assessed intensity levels of physical activity and academic achievement and test whether cardiovascular fitness mediates the association between physical activity and academic achievement. STUDY DESIGN: Cross-sectional data were gathered in Swedish 9th-grade students (n = 232; mean age = 16 years; 52% girls). School grades, pubertal phase, skinfold thickness, cardiovascular fitness, and physical activity were measured objectively. Mother's education, family structure, and parental monitoring were self-reported. Data were analyzed with linear regression analyses. RESULTS: After controlling for confounding factors, academic achievement was associated with vigorous physical activity in girls (beta = .30, P < .01; explained variance of the model 26%), which remained after inclusion of fitness (beta = .23, P < .05; explained variance 29%). The association was not mediated by fitness. In boys, academic achievement was associated with pubertal phase (beta = .25, P < .05). After inclusion of fitness, it was only associated with fitness (beta = .25, P < .05; explained variance of the model 30%). CONCLUSION: In girls, academic achievement was associated with vigorous physical activity and not mediated by fitness, whereas in boys only fitness was associated with academic achievement. Further studies are necessary to investigate the potential longitudinal effect of vigorous physical activity on academic achievement, the role of fitness herein and the implications of these findings for schools.