The aim of this study was to verify the association between children's body mass index and their mother's obesity, considering children's physical fitness as a possible moderator. Cross-sectional study developed with 1842 children and adolescents, aged seven to 17 years, from Santa Cruz do Sul-RS, Brazil. Body weight and height were assessed to determine body mass index. Cardiorespiratory fitness was determined by the 6-min walk/run test and muscular strength through the lower limb strength test. Mother's perception of obesity was self-assessed. Moderation was tested through a SPSS program extension. Results indicated that higher children's body mass index (p < 0.001) and lower levels of cardiorespiratory fitness (p = 0.001) and muscular strength (p = 0.035) were associated with mother's obesity. Likewise, higher body mass index (p < 0.001) and lower cardiorespiratory fitness (p < 0.001) in adolescents were associated with maternal obesity. Moreover, physical fitness moderates the relationship between body mass index and mother's obesity in children (cardiorespiratory fitness: β = - 0.006; 95% CI = (- 0.010, - 0.001); muscular strength: β = - 8.415; 95% CI = (- 12.526, - 4.304)) and in adolescents (cardiorespiratory fitness: β = - 0.004; 95% CI = (- 0.008, - 0.0008); muscular strength: β - 2.958; 95% CI = (- 5.615, - 0.030)).Conclusion: increasing physical fitness is an important strategy to protect youths from high body mass index, when their mothers are obese. What is Known: • Mother's obesity is associated with their children's body mass index. • Parents' obesity is associated with their children's physical fitness What is New: • Cardiorespiratory fitness and muscular strength are moderators in the relationship between mother's obesity and BMI of their children. • Children and adolescents with high cardiorespiratory fitness and muscular strength levels are protected against elevated body mass index, considering mother's obesity.