The current study examined whether moral disengagement would mediate the association between adolescents' teacher-student relationships and classroom incivility and sex and age differences in this mediation model. We also examined whether the mediating effect of moral disengagement would be moderated by negative coping styles. Participants comprised 2646 Chinese students (mean age = 13.85 years, SD = 1.45). Results indicated that teacher-student relationships were significantly and negatively associated with adolescents' classroom incivility and moral disengagement mediated this association. Furthermore, this mediating process differed between males and females as well as early and middle adolescents. Specifically, males were more likely to use moral disengagement strategies to engage in classroom incivility than females. Age moderated the association between teacher-student relationships and moral disengagement as well as moral disengagement and classroom incivility, and these two associations became stronger for early adolescents than for middle adolescents. Moreover, negative coping styles moderated the association between teacher-student relationships and classroom incivility and the association between moral disengagement and classroom incivility. The direct association between teacher-student relationships and classroom incivility became nonsignificant for adolescents with high negative coping styles. The study contributes to clarifying the mediating and moderating mechanisms underlying the effect of teacher-student relationships on adolescents' classroom incivility.