AimEvidence for a link between physical activity and mental health in young people is hampered by methodological shortcomings. Using repeat assessments of device-measured physical activity, we examined the association of within-individual variation in free-living activity over 7 years with depressive symptoms.MethodsThis was a prospective cohort study of a nationally representative sample of children born in the UK (n=4898). Physical activity was quantified using accelerometry at ages 7 and 14. The main outcome was depressive symptoms, based on the Short Mood and Feelings Questionnaire, assessed at age 14.ResultsAfter adjustment for socioeconomic status, body mass index and psychological problems at baseline, a higher level of light-intensity activity at age 7 in girls was associated with a lower likelihood of having depressive symptoms at follow-up (OR, 0.79; 95% CI 0.61 to 1.00), although no associations were observed for moderate to vigorous activity or sedentary behaviour. Girls who transitioned from low baseline activity to higher levels at follow-up experienced a lower risk of depressive symptoms (OR, 0.60; 95% CI 0.39 to 0.92) compared with the inactive reference category. Null associations were observed in boys. Participants who consistently met the current recommendation of 60 min/day of moderate to vigorous activity both at 7 and 14 years of age experienced the lowest risk of depressive symptoms (OR, 0.55; 95% CI 0.34 to 0.88).ConclusionTo prevent depressive symptoms in adolescence, policies to increase physical activity from mid-childhood may have utility.