Youth aggression is a serious global issue, but research identifying personality traits associated with aggression has focused on adults. Little is known about whether similar associations exist during adolescence; even less is known about these associations across cultures. This study examined links between personality and physical aggression in U.S. and Chinese adolescents, and tested whether temper mediates these associations. U.S. (N = 250) and Chinese (N = 199) young adolescents ([X bar] age = 13.43 years) completed self-reports describing personality, temper, and aggression. Path analyses demonstrated that temper significantly mediated associations from agreeableness and neuroticism to aggression in both samples. The mediating effect of temper was marginally stronger in the Chinese sample than in the U.S. sample, suggesting temper plays a more important role in youth aggression in China than in the United States. Findings highlight the universal role of affect in aggression and demonstrate the importance of cultural context in understanding links between personality and youth aggression.