Objectives: To determine whether, and to what extent, a relation exists between neighbourhood design and children's physical activity and sedentary behaviours in Saskatoon. Methods: Three neighbourhood designs were assessed: 1) core neighbourhoods developed before 1930 that follow a grid pattern, 2) fractured-grid pattern neighbourhoods that were developed between the 1930s and mid-1960s, and 3) curvilinear-pattern neighbourhoods that were developed between the mid-1960s through to 1998. Children aged 10-14 years (N=455; mean age 11.7 years), grouped by the neighbourhoods they resided in, had their physical activity and sedentary behaviour objectively measured by accelerometry for 7 days. ANCOVA and MANCOVA (multivariate analysis of covariance) models were used to assess group differences (p<0.05). Results: Group differences were apparent on weekdays but not on weekend days. When age, sex and family income had been controlled for, children living in fractured-grid neighbourhoods had, on average, 83 and 55 fewer accelerometer counts per minute on weekdays than the children in the core and curvilinear-pattern neighbourhoods, respectively. Further analyses showed that the children in the fractured-grid neighbourhoods accumulated 15 and 9 fewer minutes of moderate-to-vigorous physical activity per day and had a greater time spent in sedentary behaviour (23 and 17 minutes) than those in core and curvilinear-pattern neighbourhoods, respectively. Conclusion: These data suggest that in Saskatoon there is a relation between neighbourhood design and children's physical activity and sedentary behaviours. Further work is needed to tease out which features of the built environments have the greatest impact on these important lifestyle behaviours. This information, offered in the context of ongoing development of neighbourhoods, as we see in Saskatoon, is critical to an evidence-informed approach to urban development and planning.