BACKGROUND : Socio-ecological models indicate that family, school, and community environment explains children's physical activity and body weight. This study investigated whether parental perceptions of school/community-based physical activity (PA) promotion programs as well as parental and child perceptions of parental instrumental support for child PA (transportation provision) would predict child body weight. Child moderate-to-vigorous physical activity (MVPA) was hypothesized to mediate these associations. METHODS : Data of 879 parent-child dyads were collected at two measurement points: the baseline (T1) and the 7-8-month follow-up (T2). Parents were 23-68 years old (83.3% women), while children were 5-11 years old (52.4% girls). Parents and children reported their perceptions of environment, support (T1), and MVPA (T1, T2). Parental and child body weight and height were measured objectively (T1, T2). RESULTS : Path analyses indicated indirect effects of parental perceptions of school/community-based PA policies (T1) and parental perceptions of transportation provision (T1) on child body weight (T2), with child MVPA (T2) operating as the mediator. There were no direct or indirect effects of child perceptions of parental transportation provision (T1) on child MVPA or body weight (T2). Similar patterns of associations were found for the total sample and the subsample of children with overweight/obesity. CONCLUSIONS : Parental perceptions of school/community-based PA policies and transportation provision may explain changes in child MVPA and body weight. Interventions aimed at prevention of child overweight/obesity may benefit from a focus on parental transportation provision to PA facilities and parental awareness of PA promotion at local environment.