The effects of screen media activity (“SMA”) (including watching television or videos, playing video games, or using social media) on brain structure are not well understood. This study used data from the Adolescent Brain Cognitive Development (“ABCD”) study to follow 9-10 year-olds longitudinally through adolescence to examine the relationship between screen media activity and structural brain characteristics. Usable structural imaging and SMA data was available for over 4000 subjects. Analysis of this data suggests that SMA is significantly related to brain structure, with mixed consequences for psychopathology and cognitive performance. More specifically, these findings provide evidence that screen media activity (i) affects how the brain matures, (ii) specifically affects the brain areas that process sensorimotor information, and (iii) may be associated with problematic outcomes if the development of different brain areas is not well coordinated. Future investigations will need to examine how various forms of screen media activity influence specific psychopathology and cognitive functions, and how this influences changes throughout development.