This study examined the prospective links between maladaptive anger regulation and aggressive behavior in middle childhood over a 10-month period, analyzing the mediating influence of peer problems. Participants were 599 elementary school children in Germany, aged 6-10 years at Time 1 (T1) and 7-11 years at Time 2 (T2). Anger regulation at T1 was assessed via a structured behavioral observation in an anger-eliciting situation. Aggression was measured using a teacher-report questionnaire that assessed the frequency as well as the functions (reactive and proactive) of aggressive behavior at T1 and T2. Peer problems were assessed through parent reports, teacher reports, and self-reports at both data waves. At T1, maladaptive anger regulation was associated with peer problems, the frequency of aggressive behavior, and reactive but not proactive aggression. Latent structural equation modeling revealed that, longitudinally, maladaptive anger regulation indirectly predicted the frequency and functions of aggression through eliciting peer problems.