This study examined prosocial bystander behavior in an online ball-throwing game (Cyberball), toward the exclusion of immigrants and nonimmigrant peers within intergroup and intragroup contexts. Participants were British children (8- to 10-year-olds) and adolescents (13- to 15-year-olds, N = 292; female N = 144). They were an ethnically diverse low-to-middle SES sample from a South Asian, White, Black, or mixed ethnic background. Participants played the game and witnessed a victim being excluded by peers. The victim's and excluders' group membership and status were highlighted in a prototypical (i.e., majority status peers excluding a minority status victim) or nonprototypical (i.e., minority status peers excluding a majority status victim) intergroup context. In intragroup contexts exclusion involved peers from the same group (i.e., majority status peers excluding a majority status victim or minority status peers excluding a minority status victim). Prosocial bystander behavior and "verbal" reactions to the exclusion were measured. Adolescents showed more prosocial bystander behavior than children when it was an intergroup context but not when it was an intragroup context. Only adolescents showed more prosocial bystander behavior when the intergroup context was prototypical compared to nonprototypical. Verbal reactions were related to prosocial bystander behavior and, with age, individuals increasingly verbally challenged the exclusion and the motivation behind it. The findings support the Social Reasoning Developmental (SRD) approach to social exclusion by showing that from late childhood into midadolescence bystander behavior is increasingly related to group membership and group status of the excluders and victim.