In this study, we investigated the association between perceptions of the built environment and physical activity among Portuguese boys and girls. Participants were 1124 adolescents (592 girls and 532 boys) 12- to 18-years-old. Physical activity and perceptions of environmental characteristics were assessed by self-report questionnaires. Significantly more girls (p<0.01) were classified as low-active (61.5%) compared to boys (22.9%). Logistic regression analyses (adjusted for age) showed a significant association between girl's physical activity and availability of free- or low-cost recreational facilities in the neighbourhood (OR=1.44; 95% CI 0.99–2.11). Among boys, the presence of people being active in the neighbourhood was associated with higher levels of physical activity (OR=1.59; 95% CI 1.05–2.40). Perceptions of some aspects of neighbourhood environment may relate differently to participation in physical activity for boys compared to girls.