BACKGROUND Physical fitness is strongly associated with several positive health indicators among adolescents. However, its association with body mass index status is inconsistent. The aim of this study was to explore the association between overweight/obesity and physical fitness among children and adolescents. METHODS The design consisted of a cross-sectional study comprising 519 Brazilian students age 7 to 15 years. BMI status was assessed according to sex- and age-specific growth charts. Physical fitness was assessed using 8 tests: sit-and-reach, stationary long jump, 1-minute curl-up, modified pull-up, medicine-ball throw, 9-minute run, 20-m run, and 4-m shuttle-run. RESULTS Prevalence of overweight and obesity was 24% and 12%, respectively. Boys performed better than girls in all tests, except flexibility. Normal weight students performed better than overweight and obese students in all tests, except the sit-and-reach and the medicine-ball throw. Cardiorespiratory fitness had the strongest association with BMI status. The prevalence of obese subjects classified as "most fit" was less than 10%. CONCLUSIONS Higher values of body mass index were associated with declines in physical fitness, independent of age. The majority of obese children and adolescents and almost a half of those overweight were classified in the third tertile of physical fitness (least fit).