Understanding which factors are related to different kinds of aggressive behaviors in adolescents might help to improve violence-prevention programs for schools and families. Although some studies show that adolescents who are less psychologically mature tend to display more behavioral problems, few studies have been performed on the relationship between aggressive behavior and psychological maturity in adolescence, and no studies have focused specifically on indirect aggression. For this reason, the current research tests the role of psychological maturity in direct and indirect aggressiveness in a sample of 193 Spanish adolescents (49% boys and 51% girls) between 14 and 18 years old (M = 16.1, SD = 1.18). The results show that psychological maturity is related to both kinds of aggressiveness. In fact, less mature adolescents tend to show higher levels of direct aggression (r = –.22, p < .01) and indirect aggression (r = –.44, p < .01). More specifically, the dimensions of psychological maturity most related to aggressiveness are self-reliance and identity: self-reliance is the main predictor of indirect aggression (p < .01) and identity is the main predictor of direct aggression (p < .01). Moreover, overall psychological maturity is more related to indirect aggression in men than in women (p < .05), so the increase in psychological maturity implies a greater decrease of indirect aggression in men.