Importance: Understanding the mechanisms by which physical activity is associated with a lower risk of psychiatric symptoms may stimulate the identification of cost-efficient strategies for preventing and treating mental illness at early life stages. Objective: To examine neurobiological, psychosocial, and behavioral mechanisms that mediate associations of physical activity with psychiatric symptoms in youth by testing an integrated model. Design, setting, and participants: Generation R is an ongoing prospective population-based cohort study collecting data from fetal life until young adulthood in a multiethnic urban population in the Netherlands. Pregnant women living in Rotterdam with an expected delivery date between April 2002 and January 2006 were eligible for participation along with their children born during this time. Data were collected at a single research center in the Erasmus Medical Center Sophia Children's Hospital. For the current study, data were analyzed from 4216 children with complete data on both exposure and outcome at ages 6, 10, and 13 years. Data were analyzed from January 2021 to November 2022. Exposures: Physical activity was ascertained at age 6 years (visit 1) via parent report and included weekly frequency and duration of walking or cycling to or from school, physical education at school, outdoor play, swimming, and sports participation. Main Outcomes and Measures: Psychiatric symptoms (internalizing and externalizing symptoms) were assessed at age 6 years (visit 1) and at age 13 years (visit 3) using the Child Behavior Checklist. Several mechanisms were explored as mediators, measured at age 10 years (visit 2). Neurobiological mechanisms included total brain volume, white matter microstructure, and resting-state connectivity assessed using a 3-T magnetic resonance imaging scanner. Psychosocial mechanisms included self-esteem, body image, and friendship. Behavioral mechanisms included sleep quality, diet quality, and recreational screen time. Pearson correlations between physical ...