This study examined the relationships between two functions of aggression (i.e., reactive and proactive) and delinquency, including the moderating effects of parent criminality and friends’ gang membership, in a sample of 1,027 Singaporean adolescents from Grade 7 to Grade 9, with age ranging from 12 to 19 years (M = 14.10, SD = 1.15). Findings suggested that both reactive aggression and proactive aggression significantly and positively predicted delinquency (after controlling for proactive aggression and reactive aggression, respectively), with proactive aggression being a stronger predictor. Friends’ gang membership was found to moderate the relationship between reactive aggression and delinquency, and proactive aggression and delinquency, with stronger moderator effects for the latter. Those who were aggressive proactively and who had friends in a gang appear to be impacted most negatively with respect to delinquency. Parent criminality did not moderate these relationships. These findings highlight the need to effectively address the issues of child and adolescent aggression. Also, developing positive peer relations early is crucial for delinquency prevention.