Background and aims: About 1/3 of youth spend more than four hours/day engaged in screen media activity (SMA). This investigation utilized longitudinal brain imaging and mediation analyses to examine relationships among SMA, brain patterns, and internalizing problems. Methods: Data from Adolescent Brain Cognitive Development (ABCD) participants with baseline and two-year follow-up structural imaging data that passed quality control (N 5 5,166; 2,385 girls) were analyzed. Joint and Individual Variation Explained (JIVE) identified a brain co-development pattern among 221 brain features (i.e., differences in surface area, thickness, or cortical and subcortical gray-matter volume between baseline and two-year-follow-up data). Generalized linear mixed-effect models investigated associations between baseline SMA, structural codevelopment and internalizing and externalizing psychopathology at two-year follow-up. Results: SMA at baseline was related to internalizing psychopathology at year 2 (ß=0:020; SE . 0:008; P . 0:014) and a structural co-development pattern (ß=0:015; SE . 0:007; P . 0:029), where the co-development pattern suggested that rates of change in gray-matter volumes of the brainstem, gray-matter volumes and/or cortical thickness measures of bilateral superior frontal, rostral middle frontal, inferior parietal, and inferior temporal regions were more similar than those in other regions. This component partially mediated the relationship between baseline SMA and future internalizing problems (indirect effect 5 0.020, P-value 5 0.043, proportion mediated: 2.24%). Discussion and conclusions: Greater youth engagement in SMA at ages 9.10 years statistically predicted higher levels of internalizing two years later. This association was mediated by cortical-brainstem circuitry, albeit with relatively small effect sizes. The findings may help delineate processes contributing to internalizing behaviors and assist in identifying individuals at greater risk for such problems. Kulcsszavak: screen media activity, addictive behaviors, brain co-development pattern, internalizing behavior, substance use problems, addiction circuit