Background: We investigated if a physical activity program for young adults with Down syndrome was feasible using a phase II, single-blind, randomised controlled trial. Method: Sixteen adults with Down syndrome (8 women, 8 men, M age = 21.4 ± 3.2 years) were randomly assigned to an intervention, in which participants were asked to complete 150 minutes of walking each week for 8 weeks, or a control group, in which participants completed an 8-week program of social activities. The outcomes assessed by an assessor blind to group allocation included feasibility, safety, waist circumference, walking capacity, physical activity, and wellbeing. Data were analysed using analysis of covariance. Results: The participants logged an average of 175 minutes per week walking (range: 146-262 minutes). No adverse events occurred. Sample size estimation for a phase III study suggests 41 participants per group would be required to demonstrate a significant difference. Conclusions: This phase II trial provided preliminary evidence that a walking program is feasible and safe for young adults with Down syndrome with mild to moderate intellectual disability.