Background: Time spent on physical activity in elementary school has been altered to improve core academics. However, little is known about the relationship between physical fitness and academic achievement. We examined the association between physical fitness and academic achievement and investigated the influence of parental socioeconomic status and ethnicity. Methods: Participants were 542 girls and 577 boys aged 13 to 15 residing in the Danish municipality of Aalborg. A watt-max cycle ergometer test was completed to evaluate physical fitness as represented by VO[subscript 2]max (mL·kg[superscript -1]·min[superscript -1]). Academic achievement was measured 1 school year later through a series of mandatory exams within the humanities, sciences, and all obligatory defined exams. Parental income and education were drawn from nationwide registers. Linear regression models were used to investigate the association. Results: Adjusting for ethnicity and parental socioeconomic status, the effect size of the humanities was 0.08 grad/VO[subscript 2]max (95% Cl:0.05 to 0.11) for girls and 0.06 grad/VO[subscript 2]max (95% Cl:0.03 to 0.08) for boys. The effect size of the sciences was 0.09 grad/VO[subscript 2]max (95% Cl:0.05 to 0.13) for girls and 0.06 grad/VO[subscript 2]max (95% Cl:0.03 to 0.09) for boys. The effect size of the defined exams was 0.09 grad/VO[subscript 2]max (95% Cl:0.06 to 0.11) for girls and 0.06 grad/VO[subscript 2]max (95% Cl:0.03 to 0.08) for boys. Conclusion: We found a statistically significant positive association between physical fitness and academic achievement after adjusting for ethnicity and parental socioeconomic status.