Peer victimization in schools most often occurs in the presence of bystanders. When bystanders intervene on behalf of the victims, they are often successful in stopping the victimization. Defender self-efficacy (i.e., the belief in one's ability to successfully defend victims) has consistently been associated with greater defending and less passive bystanding in peer victimization. However, the lack of longitudinal research designs has resulted in a limited understanding of how these relationships develop over time. This five-wave longitudinal study involving 2507 Swedish students addressed this gap by examining longitudinal associations of defender self-efficacy with defending and passive bystanding. Participating students answered a self-report questionnaire once a year, from fourth to eighth grade. Our findings provide partial evidence for reciprocal associations among the variables. Moreover, there were more significant associations in the traditional than in the random intercept model, thus favoring between-person interpretations of the longitudinal associations. The findings highlight the importance of understanding the link between defender self-efficacy and bystander behavior of peer victimization, and that schools in their efforts in preventing school violence and bullying support students in increasing their defender self-efficacy and capacity in defending.