Background: This umbrella review aimed to summarise the evidence presented in systematic reviews and meta-analyses regarding the effect of physical activity on academic achievement of school-age children and adolescents. Methods: A comprehensive electronic search for relevant systematic reviews and meta-analyses were performed in Pubmed, Cochrane Library, Web of Science, Scopus, and Latin American and Caribbean of Health Sciences Information System, and reference lists of the included studies, from inception to May 2020. Studies were included if they were systematic reviews or meta-analyses, included school-age children or adolescents, the intervention included physical activity, and the outcome was the academic achievement. Two independent authors screened the text of potentially eligible studies and assessed the methodological quality of the studies using the AMSTAR 2 tool. Results: Forty-one systematic reviews and meta-analyses that examined the effects of physical activity on children and adolescents’ academic achievement were identified. Overall, the systematic reviews reported small positive or mixed associations between physical activity and academic achievement. From meta-analyses, it was observed that physical activity had null or small-to-medium positive effects on academic achievement. Chronic physical activity showed a medium positive effect on academic achievement, and acute physical activity did not demonstrate benefits. Conclusions: Physical activity seems not to be detrimental to school-age children and adolescents’ academic achievement, and may, in fact, be beneficial.