Objective: This study aimed to determine the association between body mass index (BMI) and functional fitness (FF) in 9- to 10-year-old Chilean children. Methods: This was an analytical and cross-sectional study with a probabilistic sample consisting of 139 children of both sexes. Anthropometric measurements, hemodynamic variables, physical activity (PAQ-C), and FF (measured by a 6-minute walk test: 6MWT) were recorded. If the distance walked was <10th percentile, FF was considered low. Means of variables between each group were compared by Student's t test, and Pearson's test was used to determine correlations between variables. Multivariable analyses were performed to identify factors associated with FF with a level of significance P < .05. All these analyses were performed with the STATA 15.0 program. Results: It was clear boys walked a greater distance than girls (P = .006); overweight subjects had lower physical performance than normal-weight subjects (P = .035). There was a significant correlation between weight (r = −0.511), height (r = 0.502), BMI (r = −0.352), waist circumference (r = −0.310), neck circumference (r = −0.204), and percentage fat mass (r = −0.310) for distance walked. Overweight (OR 2.06; 95% CI 1.6-4.8), high percentage fat mass (OR 1.84; 95% CI 1.2-3.2), and abdominal obesity (OR 1.15; 95% CI 1.1-1.7) were associated with significantly higher odds of low FF. Conclusion: Lower performance was exhibited in Chilean children for the 6MWT. Overweight, fat mass, and abdominal obesity were associated with lower FF. This study contributes empirical information to construct normality curves for the Chilean child population.