Background: Children and adolescents with high callous-unemotional traits (CU) are more likely to engage in aggressive and antisocial behaviours, such as cyber-bullying, but the relationship is not direct, as it may be influenced by other factors. Objective: In the absence of substantial supporting evidence, the purpose of this study was to investigate the moderating effect of maternal and paternal attachment, namely dependency and availability, in the relationship of CU traits with cyber-bullying. Methods: The convenience sample consisted of 331 elementary and junior high school Greek students who voluntarily completed an anonymous questionnaire assessing cyber-bullying, attachment with each parent (dependence and availability) and CU traits (callousness, unemotionality, uncaringness). Results: Cyber-bullying correlated positively with CU traits, and attachment negatively with cyber-bullying and CU traits. Boys scored higher in cyber-bullying, callousness, and uncaringness, and girls in mother availability and dependence. Mother availability, mother dependence, and father availability moderated the association between callousness and cyber-bullying. Conclusions: Mother availability, mother dependence, and father availability may buffer the linkages between callousness and cyber-bullying. While promising, longitudinal and intervention designs targeting these processes are needed to show that strengthening parent-child attachment may be an effective strategy for reducing externalized problems such as cyber-bullying.