The distinction between proactive and reactive functions of aggression is one of the most common divisions when investigating aggression among children and adolescents. To date, self-report is the least used measurement, despite existing literature supporting the view that the best informant regarding internal processes and motives are children themselves. The main aim of this study was to examine the construct and concurrent validity of a new self-report questionnaire, which aims to disentangle acts of reactive vs. proactive aggression that are most common within the daily lives of children. We examined the self-report measure among 578 children (313 girls, 265 boys, mean age 11 years, range 9–13 years). Most children (90% boys; 85% girls) reported at least one act of aggression over the last four weeks. Furthermore, the outcomes support the two-factor structure (reactive and proactive aggression) and the questionnaire showed good concurrent and discriminant validity with measures for emotional and social functioning. This study validates the use of the self-report instrument for reactive and proactive aggression and demonstrates that children can successfully distinguish between their own motives for reactive and proactive forms of aggressive behaviours.