Background <p>Both nature and physical activity practice have been identified as positive contributors to mental health and well-being. Engaging in outdoor physical activity (OPA) likely combines these benefits. However, the mechanisms through which these associations operate remain unknown. Since OPA can promote the satisfaction of the basic psychological needs of autonomy, competence and relatedness and basic psychological needs are associated with positive mental health, they could represent a mediator in the OPA-positive mental health association. The aims of this study are to 1) estimate the association between OPA and positive mental health in adolescents and 2) examine whether satisfaction of autonomy, competence and relatedness mediate this relationship.</p> Methods <p>Data from the MATCH longitudinal study were used to examine these objectives in young (14–15&#xa0;years), middle-age (15–16&#xa0;years) and older (16–17&#xa0;years) adolescents. OPA and satisfaction of the basic psychological needs of autonomy, competence and relatedness were self-reported three times per year. Positive mental health was self-reported once per year. Linear and logistic regression models (objective 1) and mediation analyses based on counterfactual definitions of natural direct (NDE) and natural indirect (NIE) effects (objective 2) were performed, adjusting for age, gender, puberty stage, and neighborhood income.</p> Results <p>No association was found between OPA frequency and positive mental health in young adolescents (OR [95% CI] = 1.10 [0.69, 1.75]); however, a positive association is observed in middle-age (OR [95% CI] = 1.99 [1.11, 3.57]) and older (OR [95% CI] = 3.40 [1.25, 10.09]) adolescents. Mediation analyses suggest that only relatedness may mediate the OPA-positive mental health association among middle-age adolescents.</p> Conclusions <p>Results indicate that OPA may relate differently to positive mental health across adolescence and that underpinning mechanisms need to be further investigated.</p>