OBJECTIVES: Using physical activity in the teaching of academic lessons is a new way of learning. The aim of this study was to investigate the effects of an innovative physically active academic intervention (“Fit & Vaardig op School” [F&V]) on academic achievement of children. METHODS: Using physical activity to teach math and spelling lessons was studied in a cluster-randomized controlled trial. Participants were 499 children (mean age 8.1 years) from second- and third-grade classes of 12 elementary schools. At each school, a second- and third-grade class were randomly assigned to the intervention or control group. The intervention group participated in F&V lessons for 2 years, 22 weeks per year, 3 times a week. The control group participated in regular classroom lessons. Children’s academic achievement was measured before the intervention started and after the first and second intervention years. Academic achievement was measured by 2 mathematics tests (speed and general math skills) and 2 language tests (reading and spelling). RESULTS: After 2 years, multilevel analysis showed that children in the intervention group had significantly greater gains in mathematics speed test (P < .001; effect size [ES] 0.51), general mathematics (P < .001; ES 0.42), and spelling (P < .001; ES 0.45) scores. This equates to 4 months more learning gains in comparison with the control group. No differences were found on the reading test. CONCLUSIONS: Physically active academic lessons significantly improved mathematics and spelling performance of elementary school children and are therefore a promising new way of teaching.