Background: Foot problems may limit physical activity in children with Down syndrome. This prospective cohort study investigated the association of foot posture, deformity, and footwear fit with activity.Method Foot posture of 50 children with Down syndrome (22 girls, 28 boys; M age = 10.5 years) was assessed using the arch index and deformities were documented. Footwear fit compared the difference in length and width of shoes with participants' feet. Physical activity was measured using accelerometers. Results: Mean arch index (0.29 ± 0.08) was indicative of flat feet. Footwear fit was too short for five participants, too long for 26 participants, and too narrow for 29 participants. Regression analysis for the 20 participants adherent with wearing an accelerometer found no association of foot posture or deformity with activity. Footwear fit was negatively associated with activity (p = .03). Conclusion: Preliminary data suggest poor footwear fit is associated with reduced physical activity in children with Down syndrome.