Adolescents with negative student--student relationships are at increased risk for bullying perpetration. Moral disengagement has been well documented as one of the main examined predictive variables of bullying perpetration. However, few studies have examined the mechanism of moral disengagement in the association between student--student relationship and adolescents' bullying perpetration. The present study examined the bidirectional associations among student--student relationship, moral disengagement, and bullying perpetration. Moreover, the present study tested the longitudinal mediating effect of moral disengagement and the moderating effect of gender. Participants were 2,407 Chinese adolescents (M[subscript age] = 12.75, SD = 0.58 at baseline). The cross-lagged panel model (CLPM) revealed that prior student-student relationship was associated with later bullying perpetration ([beta][subscript T1[right arrow]T2] = -0.11 and [beta][subscript T2[right arrow]T3] = -0.12), prior student-student relationship was associated with later moral disengagement ([beta][subscript T1[right arrow]T2] = -0.15 and [beta][subscript T2[right arrow]T3] = -0.10), and prior moral disengagement was associated with later bullying perpetration ([beta][subscript T1[right arrow]T2] = 0.22 and [beta][subscript T2[right arrow]T3] = 0.10). Furthermore, moral disengagement at Time 2 significantly mediated the association between student-student relationship at Time 1 and bullying perpetration at Time 3 ([beta] = -0.015). Gender moderated the mediating effect of moral disengagement. These findings highlight the importance of student-student relationship and moral disengagement in antibullying intervention programs.