Bullying and cyberbullying perpetration can involve cognitive processes of moral disengagement; however, there is no clear evidence about which strategies have the greatest influence on this type of behaviour. The aim of this paper was to examine which dimensions of moral disengagement were associated to bullying (off/online) and to explore the effect of gender and age. A total 1,274 students (48.6% girls, aged 11 to 17 years old) from the south of Spain were surveyed (M = 13.63, SD = 1.31). Multivariate multiple regression analyses showed that age and all moral disengagement mechanisms were associated with both offline and online bullying. Univariate regressions revealed that cognitive restructuring was the most strongly associated with both. The interaction between age and cognitive restructuring was only related to offline bullying. Simple slope analyses showed different effects for younger and older classmates at high levels of cognitive restructuring. Specific moral disengagement strategies have special significance for adolescent bullying and cyberbullying perpetration, with cognitive restructuring in particular promoting bullying perpetration in younger students. The results are discussed in relation to practical implications to prevent bullying and cyberbullying.