The Reactive Proactive Aggression Questionnaire (RPQ) is a self-report tool for assessing reactive aggression (RA) and proactive aggression (PA). This study contributes to the literature by testing the psychometric properties of the RPQ across detained boys from various ethnicities whilst using data that were gathered during clinical assessments. The factorial, convergent, and criterion validity, and the internal consistency of the RPQ scores received strong support in the total sample and across four ethnicity groups. Also, three groups of boys were identified, with the group including boys with high levels of both RA and PA including the most severe boys in terms of anger, delinquency, alcohol/drug use, and psychopathic traits, and having the highest prevalence rate of conduct disorder and substance use disorder. Together, these findings suggest that the RPQ may hold promise for assessing RA and PA in detained boys, even when confidentiality and anonymity of the information is not guaranteed.