The purpose of this study is to understand the effect of middle school students' overdependence on SNS on cyberbullying perpetration and to verify the mediating effect of depression in these relationship. The subjects of this study were a total of 346 middle school students who attend two middle school and use three youth centers located in the northern part of Seoul, and offline and online surveys were conducted from July 15 to August 30, 2020. The collected data were analyzed to multiple regression analysis using the SPSS 25.0 program. The results of this study were as follows. First, middle school students' overdependence on SNS had a statistically significant effect on depression. Second, middle school students' overdependence had a significant effect on cyberbullying perpetration. It was found that depression mediated middle school students' overdependence on SNS and cyberbullying perpetration. Middle school students who used SNS excessively showed high cyberbullying perpetration, and depressed middle school students showed high cyberbullying perpetration. These results will be used to develop basic data or programs to prevent middle school students' excessive dependence on SNS and cyberbullying perpetration.