The regular practice of physical activity contributes to weight control and improves maximal oxygen uptake (VO2max), socialization and quality of life. Aim: To determine the effect of moderate-to-vigorous physical activity on body composition and VO2max in a group of overweight school-age males. Material and Methods: We studied fifty-five overweight males aged 15.6 ± 0.7 years. Participants were divided in two groups (experimental and control groups). The intervention in the experimental group was the practice of small-sided games during 60 minutes, two days per week and during 11 weeks. At baseline and the end of the intervention, body composition was measured using bioimpedance and VO2max was calculated using the Course Navette test. Results: A 7% reduction in the percentage of body fat and a 9% increase in VO2max were observed in the experimental group (p < 0.01). No changes were recorded in the control group. Conclusions: Eleven weeks of small-sided exercises performed twice per week increased VO2max and reduced fat mass in overweight adolescents.