This study examines the mediating effect of positive attitudes toward victims and the moderating role of class coping efficacy in the relationship between prosociality and personal defender coping efficacy in cyberbullying among elementary school students. Data are drawn from the fourth wave of a nationwide survey conducted by the Blue Tree Foundation, and the sample consisted of 2,055 students in Grades 3 to 6. Prosociality, positive attitudes toward victims, personal defender coping efficacy, and class coping efficacy were assessed using self-report questionnaires. Mediation, moderation, and moderated mediation analyses are conducted. The results showed that prosociality was positively associated with personal defender coping efficacy, and this relationship was partially mediated by positive attitudes toward victims. In addition, class coping efficacy significantly moderated the association between positive attitudes toward victims and personal defender coping efficacy, such that the effect of positive attitudes was weaker in classrooms with higher levels of class coping efficacy. Furthermore, the indirect effect of prosociality on personal defender coping efficacy through positive attitudes toward victims varied depending on the level of class coping efficacy, confirming a moderated mediation effect. These findings suggest that promoting cyberbullying defense behaviors requires not only strengthening individual prosocial characteristics and victim-supportive attitudes but also enhancing collective coping efficacy at the classroom level.