Adolescent psychopathic tendencies are associated with phenotypic increases in proactive aggression. However, the extent to which an understanding of others’ affective mental states, or affective theory of mind (ToM), contributes to proactive aggression remains unknown. We examined how performance on a well-known test of affective ToM, based on cropped images of the eye region, contributes to reactive and proactive types of aggression in a mixed ethnicity sample of 80 incarcerated adolescent boys. A hierarchical regression model showed that affective ToM predicted proactive aggression over and above the influence of clinically rated psychopathic tendencies. Importantly, affective ToM was unrelated to reactive aggression. Our results suggest that being able to recognize others’ affective mental states may be an important factor in aggressing against others for personal gain. These findings have implications for interventions designed to enhance ToM in youth with conduct problems.