Objective: Justice-involved youth report high rates of adverse childhood experiences (ACEs; abuse, neglect, household dysfunction) and are at high risk for elevated behavioral health needs (i.e., substance use, psychiatric symptoms). Research with broad samples of adolescents shows ACEs predict behavioral health outcomes, yet most research on the impact of ACEs among justice-involved youth focuses on recidivism. The present study addresses this gap by examining the prospective association between ACEs and psychiatric symptoms, substance use, and substance-related problems (i.e., consequences of use) among first-time justice-involved youth. Method: First-time justice-involved youth (n = 271; 54.3% male; M age = 14.5 years; 43.5% Latinx; non-Latinx: 34.2% White, 8.6% Black, 7.1% Other, 6.7% Multiracial) and their caregivers were assessed at youth’s first court contact and 4- and 12-month follow-ups. Youth and caregivers reported youth’s exposure to ACEs through a series of instruments at baseline and 4-months (e.g., Childhood Trauma Questionnaire Short-Form; Traumatic Life Events Inventory). Primary outcomes included youth alcohol and cannabis use (Adolescent Risk Behavior Assessment), consequences of use (Brief Young Adult Alcohol Consequences Questionnaire; Brief Marijuana Consequences Scale), and psychiatric symptoms (Behavior Assessment System for Children; National Stressful Events Survey PTSD Short Scale). Results: Youth were exposed to three ACEs, on average, prior to first justice contact (M = 3). Exposure to more ACEs, particularly abuse, predicted substance use and psychiatric outcomes. Gender differences emerged for cannabis use and internalizing symptoms. Conclusions: Implications for trauma-responsive juvenile justice reform are discussed, including screening for ACEs and their sequelae at first court contact and considering the role of masculine norms. (PsycInfo Database Record (c) 2021 APA, all rights reserved)