Objectives: This study aimed to investigate the prevalence of unhealthy dietary habits by maternal educational level and how the effect of maternal education changed between 2009 and 2019. Methods: Using data from the Korea Youth Risk Behavior Survey conducted between 2009 and 2019, we assessed the prevalence of each unhealthy dietary habit (insufficient consumption of milk and fruit/vegetable, breakfast-skipping, and frequent consumption of fast food, soft drinks, and instant noodles) and the prevalence difference between maternal middle/high school and college graduate groups in four periods. The models included maternal educational level, four periods and the interaction between them, sex, and grade. In addition, we estimated the ORs of unhealthy dietary habits between the two maternal educational groups at each period. Results: Throughout the study period, unhealthy dietary habits were consistently more prevalent among mothers with lower education levels. Between 2009–2010 and 2017– 2019, the prevalence of unhealthy dietary habits increased, with a particular increase in that of frequent consumption of fast food and soft drink. The prevalence differentials between the middle school and college graduate group decreased or did not differ, while those between the high school and college graduate groups increased over time.A similar trend was observed in the relative scale. Conclusion Considering the increase in the prevalence of unhealthy dietary habits and the prevalence differentials by maternal educational level, targeted efforts are needed not only for all adolescents and their parents but also for those with low socioeconomic status to improve the dietary habits of adolescents.