The aim of the current study was to investigate how moral disengagement and defender self-efficacy were related to bystander behaviour in social and cyberbullying. Four hundred and ninety-five emerging adults completed an online survey consisting of two measures of moral disengagement, a measure of defender self-efficacy and an adapted version of the Student Bystander Behaviour Scale. Regression analyses revealed that moral disengagement for the whole sample was positively associated with pro-bully behaviour and that defender self-efficacy was positively related to defender behaviour in both the social and cyberbullying contexts. The findings revealed that in order to better explain bystander behaviours, researchers should consider multiple cognitive mechanisms involved in bullying across various contexts. This study demonstrated the necessity of investigating social bullying and cyber bullying across various developmental periods and in turn may inform intervention efforts on how to encourage individuals to defend others when confronted with various forms of bullying