Objective: The public health threats stress and adiposity have previously been associated with each other. Longitudinal studies are needed to reveal whether this association is bidirectional and the moderating factors. Methods: In the longitudinal Children’s Body Composition and Stress study, 316 children (aged 5-12 years) had measures of stress (questionnaires concerning negative life events, problem behavior, and emotions) and adiposity (body mass index, waist-to-height ratio, and fat percentage) in three waves at 1-year intervals. The bidirectionality of the association between stress and adiposity was examined using cross-lagged analyses. We tested moderation by cortisol and life-style (physical activity, screen time, food consumption, eating behavior and sleep duration). Results: Adiposity (body mass index: beta = 0.48 and fat percentage: beta = 0.18; p < .001) were associated with subsequent increased stress levels, but stress was not directly related to subsequent increases in adiposity indices. Cortisol and life-style factors displayed a moderating effect on the association between stress and adiposity. Stress was positively associated with adiposity in children with high cortisol awakening patterns (beta = 0.204; p = .020) and high sweet food consumption (beta = 0.190; p = .031), whereas stress was associated with lower adiposity in the most active children (beta =-0.163; p = .022).Conclusions: Stress is associated with the development of children's adiposity, but the effects depend on cortisol levels and life-style factors. This creates new perspectives for multifactorial obesity prevention programs. Our results also highlight the adverse effect of an unhealthy body composition on children's psychological well-being.