Bystanders’ responses play an important role in cyberaggression incidents among youth. This study examines factors differentiating cyberbystander roles as defenders or outsiders. Individual factors (gender, age, self-efficacy, and digital skills), social factors (parent, teacher, and peer mediation), contextual factors (victim’s age relative to the bystander, perceived victim’s emotional (upset) response, and bystander-victim relationship quality), and types of incident modalities are explored. Data from 736 Czech adolescents (Mage = 14.4 years, SDage = 1.69, 51.9% boys) who acted as defenders or outsiders in cyberaggression incidents in the preceding year were analyzed using hierarchical binary logistic regression. Being a defender versus an outsider was significantly associated with younger age, active peer mediation, good relationship with the victim, higher perceived victim’s emotional (upset) response, and no video modality. The study underscores the multifaceted nature of online bystander behavior, offering insights for prevention and intervention targeting specific factors to promote defending behavior in adolescent cyberaggression.