The aim of the current study was to longitudinally investigate the bi-directional relationship between moral disengagement and bullying perpetration in a sample of 1,354 students from 108 elementary classes in 69 public schools. Students participated in the study both at Time 1 (fourth grade) and around one year later at Time 2 (fifth grade). Structural equation model analyses showed that bullying perpetration at Time 1 predicted moral disengagement at Time 2, when controlling for moral disengagement stability over time. In addition, we found that moral disengagement at Time 1 also predicted bullying perpetration at Time 2, when controlling for bullying perpetration at Time 1. These findings suggest that teachers, school staff, and professionals should consider interventions that address moral disengagement when working with children involved in bullying.