Workplace bullying has negative effects on targets’ well-being. Researchers are increasingly aware that bullying occurs within social contexts and is often witnessed by others in the organization, such as bystanders. However, we know little about how bystanders’ responses influence outcomes for those exposed to bullying. In this multilevel study, involving 572 employees within 55 work groups, we explore how bystanders’ passive (e.g., inaction) and active constructive (e.g., defending the target) responses to bullying can affect targets’ somatic symptoms and work engagement. Drawing from Job-Demands Resource theory, we propose that passive and active constructive bystanders can worsen or buffer bullying’s effects on these well-being outcomes respectively. Specifically, we propose that passive bystanders can act as further demands for targets to cope with, leading to demand accumulation, while active constructive bystanders can act as resources. We found that exposure to workplace bullying was associated with somatic symptoms and low work engagement. The number of passive and active constructive bystanders in the target’s work group moderated the relationship between exposure to bullying and engagement. In particular, with larger numbers of passive bystanders the negative relationship of bullying exposure with engagement strengthened. Conversely, with a higher number of active constructive bystanders, bullying’s negative relationship with engagement was mitigated. However, there was no moderating effect for somatic symptoms. This study contributes as the first empirical test of whether bystander behavior shapes the consequences of bullying for targets and provides a novel, group-level perspective to the bullying bystander literature.