The aim of this study was to investigate possible interactive links between theory of mind (ToM), moral disengagement and relational aggression, using a moderated mediation analysis, with gender as a moderator, in a sample of 120 Greek preadolescents. Results indicated that relational aggression was significantly positively associated with moral disengagement and negatively with ToM. Moderated mediation analyses indicated that boys with deficient ToM were more likely to morally disengage from their actions which in turn resulted in relational aggression, while poor ToM was directly linked to relational aggression only for preadolescent girls. Moral disengagement had a direct effect on relational aggression only for boys, while ToM was found to partially mediate the relationship between moral disengagement and relational aggression only for girls. The results emphasize that the co-morbid effects of socio-cognitive factors should be taken into consideration when relational aggression is explored.