Background and Study Aim. Obesity is a metabolic disease characterised by excessive fat accumulation. Indonesia ranks second after Singapore with the largest number of obese adolescents. The study aimed to assess the impact of physical activity, eating behavior, and sleep quality on Body Mass Index (BMI) in adolescents. Materials and Methods. This study was quantitative, with an ex post facto design. The study included 231 students (136 males, 95 females) aged 16-19 years, each with a Body Mass Index (BMI) ≥ 25.1. Participants were healthy and consented to the study by completing a screening questionnaire. The analysis was performed using regression techniques in the Statistical Package for Social Sciences (SPSS) version 21 software. Results. The selected regression model is feasible and demonstrates that physical activity, diet, and sleep quality collectively influence students' BMI (p-value < 0.001). Furthermore, the analysis shows that physical activity (p-value < 0.001), diet (p-value = 0.019), and sleep quality (p-value < 0.001) independently affect students' BMI. Conclusions. We suggest that schools need to hold counseling programs in collaboration with relevant health workers to conduct counseling with the prevention of malnutrition, especially obesity. Provide additional tasks in the form of physical activity that students must do at home with parental supervision and the results will be reported to the teacher as an additional task value. Parents should also provide supervision of eating behavior and sleep patterns. For future researchers to be able to reveal other variables that can affect student BMI, because our report found only 49.90%.