OBJECTIVE: To evaluate concurrent and longitudinal associations between psychosocial functioning and physical activity in adolescent and young adult survivors of early childhood cancer. METHODS: Adolescent survivors of early childhood cancer (diagnosed before age four) participating in the Childhood Cancer Survivor Study completed the Coping Health and Illness Profile-Adolescent Edition (CHIP-AE; n = 303; mean age at survey: 17.6 years). A subset of these survivors (n = 248) completed a follow-up survey an average of 6.0 years later (range: 4-10). Logistic regression identified associations between psychosocial functioning in adolescence and physical activity levels in adolescence and young adulthood. RESULTS: Survivors reported low physical activity as adolescents (46.1% scored below CHIP-AE cut-point) and young adults (40.8% below Centers for Disease Control guidelines). Poor physical activity during adolescence was associated with female sex (OR = 2.06, 95% CI, 1.18-3.68), parents with less than a college education (OR = 1.91, 95% CI, 1.11-3.32), previous treatment with cranial radiation (OR = 3.35, 95% CI, 1.69-6.88), TV time (OR = 1.77, 95% CI, 1.00-3.14), and limitations of activity due to health or mobility restrictions (OR = 8.28, 95% CI, 2.87-30.34). Poor diet (OR = 1.84, 95% CI, 1.05-3.26) and low self-esteem (OR = 1.80, 95% CI, 0.99-3.31) during adolescence were associated with lower odds of meeting Centers for Disease Control physical activity guidelines in young adulthood. CONCLUSION: These findings provide targets for future interventional studies to improve physical activity in this high-risk population.