Background: Overnutrition is defined as abnormal or excessive accumulation of fat in the adipose tissue that may affect the health status of individuals explained by overweight/obesity. Globalization, improving economic conditions, decreased physical activities, urbanization and changing dietary habits in developing countries are reported as some of the factors for the rapid increase in overnutrition for adolescents. Studies on determinants of overnutrition among adolescents are scarce in Ethiopia, specifically in Debre Berhan town. Objective: The main aim of this study is to identify the determinants of overnutrition among secondary and preparatory school adolescents in Debre Berhan Town, North Shewa Zone, Amhara Region, Ethiopia, 2021. Methods: Institutional based unmatched case-control study was conducted among 285 adolescents from March 18 to April 20 2021. The data was collected using interviewer-administered and pretested tools. A survey was conducted to identify cases and controls. The cases and controls were selected using simple random methods. Data was entered using Epi-data 4.2 software and exported to Statistical Package for Social Science version 25. Bivariable and multivariable logistic regression analysis was performed. World Health Organization AnthroPlus software was used to analyze anthropometric data into body mass index for age with z-score. Adjusted odds ratio with 95% confidence interval and p value <0.05 was considered as statistical significance. Result: Two hundred seventy nine adolescents (93 cases and 186 controls) were participated which made the response rate of 98% for both cases and controls. Significant association was found on earning an average family income of ≥10000 Ethiopian Birr (AOR=2.67; 95% CI: 1.214-5.9), eating habit while reading (AOR=3.87;95% CI:1.95-7.686),sedentary behavior, (AOR =2.52; 95%CI: 1.278-4.97), vigorous type of physical exercise for <75 minutes per week (AOR=2.38; 95%CI: (1.149-4.92) and <6 sleeping hour per day(AOR=5.68;95%CI:2.08-15.48). Conclusion and recommendation: Determinants of overnutrition were increased family income level, sedentary behavior, reduced physical activity, eating habit while reading and sleeping time. Most of the determinants could be preventable, collaborative health education interventions such as emphasizing on the factors of overnutrition are essential. Keywords: Determinants, overnutrition, case control, Debre Berhan