Although there is a growing interest in girls’ relational bullying, the current study was the first to examine whether classroom collective efficacy to stop relational aggression and classroom moral disengagement in relational aggression were associated with girls’ relational bullying perpetration and relational victimization. Self-reported survey data were collected from 871 Swedish schoolgirls (M=11.54, SD=0.32) from 111 classrooms. Multilevel analysis found that older classmates and girls with a foreign ethnic background were more inclined to engage in relational bullying. When classroom level variables were added, collective moral disengagement in relational aggression was positively associated with girls’ relational victimization and relational bullying, whereas collective efficacy to stop relational aggression was negatively associated with girls’ relational bullying.