Introduction: in soccer, agility is a relevant parameter that is defined as the ability to change the direction of the body rapidly and is a result of a combination of strength, speed, balance and coordination. Objective: to demonstrated the reliability and validity of the modified Barrow test in soccer players aged 10 to 14 years, analyze its relationship with age, maturation, experience and body size and test the effectiveness of the protocol for distinguishing performance changes during a 10-month competitive season. Methods: fifty-one adolescent Basque soccer players were enrolled. Age, maturation, experience, body size, vertical jump and 15-m sprint were measured. The reliability of the test was analyzed using replicate tests in a subgroup (n = 34). Another subgroup of 33 players (n =18, under-11, 10.4 ± 0.3 years; n = 15 under-13, 12.0 ± 0.8 years) was evaluated at the beginning and the end of the season. Results: the reliability of the test was good (change in mean = -0.5%, 95% CI -1.2 to 0.2, SD=0.14; coefficient of variation = 0.9%, 95% CI - 0.7 to 1.5, SD=0.25). Chronological age and adiposity were significant predictors of agility performance (P<0.01). An improvement in performance was observed over a 10-month soccer season, with a significant group effect (P<0.01). Conclusion: the agility test demonstrated logic and validity, and proved to be a reliable and objective instrument for assessing adolescent soccer players.