The purpose of this study was to examine the effects of adolescents’ school adjustment on depression in their transitional period and to find the moderating effects of parental support, parental monitoring, and peer support on the relationship between school adjustment and depression. The participants were 550 first graders in three middle schools in Incheon Metropolitan City and Kyung-gi Province. The major findings of this study were as follows: First, adolescents’ school adjustment had significant effects on depression. The lower the level of school adjustment was, the higher the level of depression was. Second, parental support, parental monitoring, and peer support had moderating effects on the relationship between school adjustment and depression. Parental support decreased the depression of adolescents who were at a lower level of adjustment to peer relationship or school class. Parental monitoring decreased the depression of adolescents who were at a lower level of adjustment to school adjustment. On the contrary, parental monitoring increased the depression of adolescents who were at a higher level of adjustment to school adjustment. Peer support decreased the depression of adolescents who were at a lower level of adjustment to school class.