To date, research has rarely addressed the association between individuals’ past cyber victimization and their position taking as bystanders in offline bullying situations, much less the psychosocial mechanism underpinning that association. By applying a social-ecological diathesis–stress model, the present study marked an initial effort to examine the direct and indirect relationships between past cyber victimization and two types of offline bystander behavior—probullying and defending—considering the possible mediation effect of domain-specific self-concept and the moderation effect of teacher and peer support among children in China. Structural equation modeling and group comparison analysis were conducted in a sample of 1,706 primary and secondary school students aged 8–17 years old (M = 11.69, 44% girls). The results indicated that cyber victimization positively related to offline probullying behavior but negatively related to offline defending behavior. Emotional, behavioral and social self-concept fully mediated cyber victimization’s effects on offline probullying behavior, whereas behavioral, social, and physical self-concept partially mediated those effects on offline defending behavior. Last, both teacher and peer support moderated the effects of cyber victimization and domains of self-concept on offline bystander behavior via certain paths. Findings recognize the dynamic, fluid nature of children’s involvement in bullying across roles and over time. Findings also highlight the importance to differentiate the specific mediation effect of domains of self-concept and the moderation effect of teacher and peer support. Both contributions are expected to enrich empirical and theoretical perspectives on bullying and provide insights for bullying prevention and intervention programs.