Although multiple cross-sectional studies have supported moral disengagement and school climate being associated with bullying perpetration, few have examined how these 3 factors interact with each other over time. Guided by the social-ecological theory and social-cognitive theory, a multilevel approach was used to examine the longitudinal associations between students' moral disengagement and bullying perpetration and the moderating effect of students' perceptions of school climate in the longitudinal association across 18 months. The main effects of moral disengagement and perceived school climate and the effects of their interaction on bullying perpetration were explored at the between-person, within-person, and across between-person and within-person levels. Participants were 2,997 adolescents (Mage = 14.9 years, SD = 1.53; 49.7% male) from 6 secondary schools in Southwest China. At the between-person level, students with higher moral disengagement and more negative perceptions of school climate perpetrated more bullying than those with lower moral disengagement and more positive perceptions of school climate. At the within-person level, students with high moral disengagement and negative perceptions of school climate had high levels of bullying perpetration over time. Moreover, significant between-person, within-person, and cross-level interactions revealed that the association between moral disengagement and bullying perpetration was weaker and nonsignificant for students with more positive perceptions of school climate. Findings indicated a buffering effect of students' perceptions of positive school climate in the longitudinal association between moral disengagement and bullying perpetration. Implications for school-based individualized bullying intervention were discussed. (PsycINFO Database Record (c) 2020 APA, all rights reserved).