Background: Individuals with Down syndrome (DS) are less physically active than the general population, but limited data on objective physical activity patterns in adults with DS are available. Methods: Free-living physical activity was measured by waist-worn accelerometry in adults with DS from 2018 to 2020. Data were analysed using descriptive statistics, bivariate and regression analyses. Results: Fifty-two subjects provided valid accelerometer data, with 46% male and a majority (75%) overweight/obese. Median (± standard deviation) daily sedentary time was 393.5 ± 216.6 min, light activity was 150.7 ± 85.5 min, moderate activity was 6.3 ± 13.5 min, and vigorous activity was 0 ± 9.8 min. Subjects had 10.3 ± 4.8 sedentary bouts per day lasting on average 30.7 ± 62.0 min. Median daily step count was 3,050 ± 1,988. Adults living alone had more steps and light physical activity in adjusted analyses. Conclusion: Adults with DS engage in little health-enhancing physical activity and do not meet current physical activity recommendations in the United States.