The purpose of this study was to examine reactivity to accelerometer measurement among adolescents with autism spectrum disorder (ASD). A sample of 23 adolescents with ASD (aged 15.00 ± 1.57 years old; 17 boys) wore triaxial accelerometers for at least 8 h per day for seven consecutive days. Descriptive statistics, including arithmetic means and standard deviations, as well as analysis of covariances with repeated measures (ANCOVAs) were conducted, controlling for participant body mass index and gender. While differences were not statistically significant, they exceed reactivity-based recommendations and have implications for future research with adolescents with ASD. The inverse reactivity pattern among adolescents with ASD is a unique finding that has important implications for research in this area.