Objectives This study reveals the mediating effect of academic engagement in the effects of middle school students smartphones dependence and grit on academic achievement. Methods The subjects of this study were 2,500 first-year middle school students from the data for the first year of the Korean Children and Youth Panel Survey 2018. Data processing was performed using the SPSS 23.0 program for frequency analysis, descriptive statistics analysis, correlation analysis and mediated regression analysis. Results First, smartphone dependence has a negative correlation with academic achievement. and grit showed a positive correlation with academic achievement. Second, the mediating effect of academic engagement was found in the relationship between smartphone dependence and academic achievement. Third, the mediating effect of academic engagement was found in the relationship between grit and academic achievement. Conclusions Through the study results, it was discussed educational interventions to increase the academic achievement of middle school students in the relationship between smartphone dependence, grit, academic enthusiasm, and academic achievement.