The objective of the present cross-sectional study was to quantify the effect size of overweight on the results of speed-agility in adolescents and to report percentile tables based on sex, age and BMI.Was hypothesized that the effect size obtained from the difference between normal-weight and overweight youth would be equal to or greater than the differences between sexes within the same age group.A total of 10.439 Spanish adolescents aged 13.72 ± 1.42 years from 42 secondary schools participated in the research. Speed-agility was evaluated using the 4x10 m test.The effect size was analysed using the adjusted Hedges’ ð. The findings regarding sex show that 92.74% and 86.98% of normal-weight and overweight girls, respectively, attained lower speed-agility than boys.With regard to body composition, the percentages are also high, but they are a bit lower than those for sex. So, 77.26% and 70.52% of overweight boys and girls, respectively, showed lower speed-agility than their normal-weight peers.The differential effect size between boys and girls is higher than between normal-weight and overweight adolescents in speed-agility. In spite of this, the results suggest that BMI should be taken into account in assessing the speed-agility of young people.