The purpose of this study was to examine the longitudinal relationship between attention-deficit/hyperactivity disorder (ADHD) symptoms, emotion regulation (ER) ability, and depressive symptoms within a diverse community sample of 277 youth, ages 9–12 (56 % male). Participants were drawn from a larger study examining adolescent risk behaviors, and completed annual assessments over 3 years. Youth ADHD symptoms were assessed at Time 1 (T1) using the parent-reported Disruptive Behavior Disorders Rating Scale, ER was assessed with the parent-reported Emotion Regulation Checklist at Time 2 (T2), and youth depressive symptoms were assessed using the self-reported Revised Child Anxiety and Depression Scales at Time 3 (T3). Analyses examined T2 ER as a mediator between T1 ADHD symptoms (including the unique contributions of inattentive [IA] versus hyperactive/impulsive [HI] symptoms) and T3 depressive symptoms. Structural equation modeling (SEM) indicated the path model specified provided an excellent fit to the data. Tests of indirect effects suggested that T2 ER appears to be a significant mechanism that underlies the relationship between T1 ADHD and T3 depression, even when accounting for T1 oppositional defiant and depressive symptoms. Furthermore, while both T1 IA and HI symptoms had significant indirect effects on T3 depression through the mechanism T2 ER, HI proved a more robust predictor of T2 ER than IA. Results of this prospective study support cross-sectional findings pointing to ER as a potential mechanism linking ADHD and depressive symptoms in youth. Clinical implications and future directions are discussed.