An extensive body of research exists on environmental influences on weight-related behaviours in young people. Existing reviews aimed to synthesize this body of work, but generally focused on specific samples, behaviours or environmental influences and integration of findings is lacking. Hereto, we reviewed 18 reviews representing 671 unique studies, aiming to identify what environmental factors do and do not affect physical activity and dietary behaviours in children and adolescents. Eleven reviews focused exclusively on physical activity, six on diet, and one review focused on both physical activity and dietary behaviours with only small overlap in included studies. Physical activity was more consistently related to school and neighbourhood characteristics than to interpersonal and societal environments. In contrast, interpersonal factors played a pronounced role in dietary behaviours; no school, neighbourhood or societal factors were consistently related to dietary behaviours. This review of reviews adds to the literature by providing a comprehensive synthesis of factors related to physical activity and dietary behaviours that could be targeted in interventions. Moreover, by identifying factors that are unrelated to physical activity and dietary behaviours, this review may help to narrow the scope of future studies and environmental interventions.