Drug and alcohol use before the age of 14 is associated with adverse outcomes over the life course. While previous studies have identified numerous sociodemographic characteristics associated with youth substance use initiation, few have examined the relationship between behavioral characteristics, such as childhood aggression, and substance use initiation in adolescence. This longitudinal study consisted of 2985 children from the Future of Families and Child Wellbeing Study. Aggression was measured using primary caregiver report when the children were about the age of 9 and cigarette, alcohol, and marijuana use was measured using the child report when the children were age 9 and age 15. Separate multivariable Poisson regression models were fitted for each substance use initiation outcome. Childhood aggression was positively associated with the initiation of cigarette and marijuana use in adolescence (aRR = 2.3 [95% CI = 1.5,3.4] and aRR = 1.3 [95% CI = 1.1,1.6], respectively). Childhood aggression was not associated with adolescent alcohol use initiation (aRR = 1.2 [95% CI = 0.9,1.5]). The presence of aggressive behavior in childhood was associated with the initiation of cigarette and marijuana use in adolescence. These results may be used to identify children at higher risk of cigarette and marijuana use, who may benefit from additional monitoring for substance use initiation.