Objectives: The purpose of this study was to evaluate the linear and curvilinear rela-tionship between body mass index (BMI) and physical fitness in children andadolescents.Methods: Participants were 4567 (2505 girls) children aged 6–16 years. Weight andheight were measured and BMI was calculated and adjusted for age and sex usingWHO z-scores. Physical fitness was evaluated with the following tests: Yo-Yo test,standing long jump, seated 2-kg medicine ball throw, and 20-m dash. Participantswere grouped into two groups according to their maturity status, estimated as yearsfrom peak height velocity. Associations were determined with linear and nonlinearquadratic regression models.Results: The nonlinear quadratic regression coefficient was significant for the 20-mdash among girls of both maturity status levels, and in maturity status level 1 boys;for the standing long jump among boys of both maturity status levels, and in maturitystatus level 1 girls. The Yo-Yo test was only significant for maturity status level 1boys. For the medicine ball throw, the linear regression coefficient was significant forboth maturity status levels in both sexes. Almost all physical fitness items wereobserved to have meaningfully large nonlinear relationships with BMIz, but theywere not all significant due to the small sample size, especially in maturity statuslevel 2.Conclusion: The association between BMI and physical fitness is nonlinear in themajority of its components, and those with lower and higher BMI had poorer fitness.