ObjectivePhysical activity supportive environments have the potential to promote health-related fitness in adults. However, the extent to which neighbourhood built characteristics promote health-related fitness via physical activity has received little research attention. Therefore, our objective was to estimate the indirect and direct effects between neighbourhood built characteristics and health-related fitness mediated by physical activity. MethodsUsing cross-sectional data collected between 2014 and 2019, we merged neighbourhood built characteristics, physical activity, and health-related fitness variables, derived from two Canadian national databases. Using these data, we estimated sex-stratified covariate-adjusted path models (males: n = 983 to 2796 and females: n = 962 to 2835) to assess if accelerometer-measured light, moderate, and vigorous intensity physical activity mediated associations between objectively measured neighbourhood built characteristics (intersection density, dwelling density, points of interest, and transit density) and health-related fitness (grip strength, jump height, V̇O2max, and flexibility). Across 16 sex-specific models, we estimated 48 indirect and 16 direct effects. ResultsConcerning significant associations, for males we found that 16.6% of indirect and 18.8% of direct were negative and 4.2% of indirect and 0% of direct were positive. For females, we found that 12.5% of indirect and 0% of direct were negative and 0% of indirect and 25% of direct effects were positive. ConclusionsIndividual Canadian Active Living Environment built characteristics are positively associated with moderate-intensity physical activity and negatively associated with light-intensity physical activity. Further, associations between activity friendly neighbourhood characteristics and health related-fitness may be distinct from physical activity.