Cyberbullying has become increasingly prevalent, difficult to detect, and harmful to its victims. Whereas correlates of cyberbullying victimization and perpetration have been studied extensively, there has been less research on the critical role that bystanders to cyberbullying instances can play. This study explored the extent to which Big Five personality traits, social dominance orientation, narcissism, moral disengagement, self-control, and cyberbullying severity level are related to bystander behavior in cyberbullying situations. Adults in the U.S. took part in an online survey in which they were presented with a series of 12 simulated social media interactions in the form of screenshots that involved exchanges between two social media users. Each screenshot depicted one of three distinct levels of cyberbullying severity: none, low severity, and high severity. For each screenshot, participants were asked to report the likelihood that they would respond in a range of ways as a bystander. Participants then completed a series of individual difference scales. The results indicated that as the severity of the cyberbullying depicted in a screenshot increased, bystanders were more likely to support the victim, flag the post, and confront the bully, and less likely to be passive observers or support the bully. Higher levels of extraversion, conscientiousness, and agreeableness were associated with a lower likelihood of remaining a passive observer, while social dominance orientation and moral disengagement were positively correlated with bystander interaction in support of the bully. Additionally, agreeableness and extraversion were positively correlated with the likelihood of supporting the victim; and agreeableness was positively correlated with the likelihood of confronting the bully. No significant relationship was discovered between self-control, narcissism, and cyberbystander behavior. This research offers experimental validation for the predictive value of both cyberbullying severity and individual differences for understanding diverse forms of cyberbystander behavior.