This study examined factors that increase or decrease the support a bystander offered to a victim of cyberbullying. Possible determinants of supportive behaviour were analyzed using a four-step hierarchical regression analysis on data from 156 Czech children (12–18 years old; M = 15.1; 54% females) who witnessed their schoolmates being victims of cyberbullying. Among individual characteristics, only a general tendency toward prosocial behaviour was a positive predictor of supportive behaviour. Other factors such as age, gender, self-esteem, and problematic relationships with peers had no effect. Among contextual factors, existing relationships with the victim, upset feelings evoked by witnessing victimization, and direct requests for help from the victim triggered supportive behaviour, while strong relationships with the bully inhibited it. Fear of intervening played no role. The practical implications of the findings are discussed with regard to the roles of the emotional response of the bystander and direct requests for help from the victim in cyberbullying interventions. Copyright © 2012 John Wiley & Sons, Ltd.