The purpose of this study is to determine the relationship between knowledge and attitude about fast food with obesity on teenagers in BanjarbaruCity, South Kalimantan Province. This study is observational analytic with cross-sectionaldesign. The population in this studywerejunior high school students in Banjarbaru City, school sample taken by simple random sampling with sample obtained 150 people. The results using chi-squaretest shows significantrelationship between knowledge and obesity on teenagers and significant relationship between attitude and obesity on teenagers. Theteenagers are advised to reduce consumption of fast foods. Fast food is not good for health and can lead to obesity and other degenerative diseases. Schools need to provide information to students about healthy eating and providing books on healthy eating at the Library.In addition, schools must ensure the type of food sold in the school canteen is healthy food and to the parents for more attention to the food intake of their children. Keywords: Knowledge, Attitude, Fast Food, Obesity.