["Child and Adolescent Mental Health, EarlyView. ", "\n\nBackground\nPhysical activity and sedentary behavior are associated with adolescent mental health. However, prior studies have not assessed whether these associations differ across varying levels of mental health severity. This study uses objectively measured physical activity and sedentary behavior to examine their associations with adolescent mental health and to determine how these associations vary across the distribution of mental health symptoms.\n\n\nMethods\nThis longitudinal prospective study examined data from the Adolescent Brain Cognitive Development (ABCD) Study, a large cohort of children and adolescents recruited from 21 study sites in the United States. This analysis included 5640 participants whose physical activity and sedentary behavior data were recorded using Fitbit wearable devices when aged 11–12 years. Outcomes were internalizing and externalizing symptoms assessed a year later by the caregiver using the Child Behavior Checklist.\n\n\nResults\nMore daily steps (b = −0.43, 95% CI [−0.58, −0.29]), longer moderate‐to‐vigorous physical activity (MVPA) (b = −0.34, 95% CI [−0.47, −0.22]) and less sedentary time (b = 0.32, 95% CI [0.15, 0.48]) were each associated with fewer internalizing symptoms a year later. The associations were stronger for youth at higher internalizing symptom quantiles. Limited or no effects were found for externalizing symptoms. Sex differences were also observed.\n\n\nConclusion\nObjectively measured daily steps, MVPA, and sedentary behavior are prospectively associated with adolescent internalizing symptoms. The associations differ substantially among adolescents with varying levels of symptom severity and between sexes. Findings may inform targeted physical activity interventions to enhance adolescent mental health.\n\n"]