Aim To evaluate the effects of a physical activity programme on sedentary behaviour and physical activity in ambulant individuals with Rett syndrome (RTT). Method In this multicentre randomized waitlist-controlled trial, we recruited 43 ambulatory individuals with RTT in Australia and Denmark. Adequate baseline data were obtained from 38 participants (mean age 20 years, range 6-41, SD 10 years 6 months, one male). All completed the trial. Participants received 12 weeks of usual care (n = 19) or a goal-based, telehealth-supported programme in which activities occurred in their familiar environments (n = 19). Sedentary time and daily steps were assessed at baseline, post-test, and 12-week follow-up. The data analyst was blinded to group allocation. Results Sedentary time decreased in the intervention group by 2.7% (95% confidence interval [CI] -6.0 to 0.6) and increased in the control group by 1.3% (95% CI -4.8 to 7.4). Intervention and control groups increased the number of their steps per day by 264.7 (95% CI -72.2 to 601.5) and 104.8 (95% CI -178.1 to 387.7) respectively. No significant differences were found on any outcomes at post-test. There were three minor adverse events. Interpretation A goal-based telehealth intervention seemed to produce small improvements in physical activity for individuals with RTT. Families require more support to increase these individuals' extremely low physical activity levels.