This study aimed to validate measures of sedentary time in individuals with Rett syndrome. Twenty-six individuals (median [IQR] age 16.0 (9.4-20.6) years) wore an activPAL accelerometer during video-taped activities and agreement was determined between sedentary time determined by the activPAL and observation. For 11 individuals (median [IQR] age 14.5 (11.5-25.6) years), linear regression was used to determine the relationship between sedentary time recorded on the modified Bouchard activity record diary card and measured using the activPAL. In comparison to observation, the activPAL accurately measured duration of sedentary time with a mean difference (limit of agreement) of -1.0 (6.3) minutes. The duration of Bouchard activity record downtime accounted for 73% of the variance of sedentary time measured by the activPAL (coefficient 0.762, 95% CI 0.413 to 1.111). These data provide clinicians and caregivers with capacity to investigate strategies that would aim to increase activity in the nonexercise component of the activity continuum.