Introduction: Overweight and obesity constitute one of the main health concerns in adolescents. Having a sufficient level of knowledge represents an option to reduce risks. The objective of the study was to evaluate the correlation between knowledge, attitudes and practices and body mass index (BMI) in Peruvian adolescents. Material and Methods: A non-experimental cross-sectional study was carried out in 242 adolescents aged 11 to 18 years. To determine knowledge, attitudes and practices on healthy eating, a questionnaire was developed according to the criteria proposed by the Food and Agriculture Organization of the United Nations (FAO). In addition, the body mass index for age (BMI/Age) was calculated according to the recommendations of the World Health Organization (WHO). Data analysis was done using IBM SPSS version 24 statistical software. Results: The proportion of women who demonstrated a high level of knowledge, favorable attitudes and adequate practices, was higher in relation to men (76.5% vs. 75.0%), (75.5% vs. 70.7%) and 72.5% vs. 67.9%, respectively. However, there were no significant differences (p>0.05). The proportion of men who were overweight and obese was 25.7% and 12.1%, respectively. Finally, a statistically significant positive correlation was found between BMI/Age with the level of knowledge, attitudes and practices about healthy eating in adolescents (rho=0.137; p=0.033), (rho=0.160; p=0.013) and (rho=0.133; p=0.039), respectively. Conclusions: The findings suggest that adolescents, especially boys, should be the focus of integrated nutritional intervention programs, with the purpose of reducing the prevalence of overweight and obesity.