Background: Technology has provided a new way of life for adolescents. Strategies aimed at improving health behaviors through digital platforms can offer promising results. However, since peers can modify behaviors related to food and exercise, studying digital interventions based on peer influence to improve weight status of adolescents is important. Objective: To assess an eHealth app's effectiveness in adolescent population improvements in age- and sex-adjusted BMI percentiles. The study also examined social relationships of adolescents pre- and postintervention, identifying group leaders to study their profiles, eating, physical activity habits, and app usage. Methods: BMI percentiles were calculated following World Health Organization guidelines. Participants' diets and physical activity levels were assessed using the KIDMED questionnaire and PAQ-A. Social network variables were analyzed using SNA methodology, with reciprocal friendships used to compute the "degree" measure for centrality. Results: The sample included 210 individuals in the intervention group and 91 in the control group, with a 60.1% participation rate. Adolescents in the intervention group modified their BMI toward the 50th percentile, improving diet and increasing social network postintervention. Group leaders were also leaders in physical activity and app usage.