["Journal of Child Psychology and Psychiatry, EarlyView. ", "\n\nBackground\nOutdoor play may offer developmental opportunities that support good mental health. Cross‐sectional data indicate that more outdoor play is associated with better mental health, particularly internalising problems (e.g. anxiety and depression), but longitudinal research is lacking. We used a longitudinal cohort dataset to evaluate whether outdoor play at 2–4 years predicted children's mental health trajectories into middle childhood.\n\n\nMethods\nData were from the Growing up in Scotland dataset. Parents/caregivers were asked how many days their child had played outside over the previous week when children were aged ~2, 3 and 4 years. When the children were aged ~4, 5, 6 and 8 years, parents/caregivers completed the Strengths and Difficulties Questionnaire measure of child mental health. We used growth mixture modelling (GMM) to identify trajectories over time for internalising and externalising symptoms separately. Next, we used multilevel multinomial regression to evaluate whether early outdoor play predicted trajectory classification.\n\n\nResults\nThe analysis sample included 4,151 children. A three‐class solution was optimal for internalising and externalising symptoms. The three trajectory groups differed on both symptom level at age 4 and slope as follows: normative (low with stable trajectory); increasing (moderate with increasing trajectory); decreasing (high with decreasing trajectory). After controlling for covariates, outdoor play significantly predicted trajectory class; children who played outdoors more often as preschoolers had significantly decreased odds (p < .001) of belonging to the increasing or decreasing trajectory groups, relative to the normative group, for internalising (increasing: OR = 0.92, decreasing: OR = 0.88) and externalising (increasing: OR = 0.94, decreasing: OR = 0.94) symptoms.\n\n\nConclusions\nThis study uses nationally representative longitudinal data and provides the first evidence that early outdoor play predicts mental health trajectories across childhood. A key limitation is reliance on parent‐report measures. The findings indicate that increasing outdoor play may be a useful public health approach to reduce child mental health problems.\n\n"]