The purpose of the present study was to determine the relationship between physical fitness and academic achievement in a sample of primary-school children. Data of 116 sixth-grade primary-school students (68 girls, 48 boys) from Barcelona (Catalonia, Spain) with a mean (SD) age of 11.4 years (0.4) was analyzed. To determine academic achievement, proficiency scores in the subjects Catalan language, Spanish language, English language, and Maths were derived from the Test de Compètencies Bàsiques. Physical fitness was assessed using the Cooper test (aerobic capacity), 20-m sprint test (linear velocity), and 4 × 10-m test (agility). The results showed that participants with a higher performance in the Cooper and 20-m sprint tests had significantly better academic achievement in the assessed subjects (p < 0.05; d range = 0.93 to −2.98). The findings of this investigation are interesting from a curricular perspective since a positive association between academic achievement and physical fitness indicates a strong reason to raise the physical education status in educational systems, a subject that has traditionally been considered less important by the educational community.