Objective: To determine the reliability and validity of the Physical Activity Scale for Individuals with a Physical Disability (PASIPD) in adults with severe traumatic brain injury (TBI) and estimate the proportion of the sample participants who fail to meet the World Health Organization guidelines for physical activity. Design and Participants: A single-center observational study recruited a convenience sample of 30 community-based ambulant adults with severe TBI. Protocol: Participants completed the PASIPD on 2 occasions, 1 week apart, and wore an accelerometer (ActiGraph GT3X; ActiGraph LLC, Pensacola, Florida) for the 7 days between these 2 assessments. Results: The PASIPD test-retest reliability was substantial (intraclass correlation coefficient = 0.85; 95% confidence interval, 0.70-0.92), and the correlation with the accelerometer ranged from too low to be meaningful (R = 0.09) to moderate (R = 0.57). From device-based measurement of physical activity, 56% of participants failed to meet the World Health Organization physical activity guidelines. Conclusion: The PASIPD is a reliable measure of the type of physical activity people with severe TBI participate in, but it is not a valid measure of the amount of moderate to vigorous physical activity in which they engage. Accelerometers should be used to quantify moderate to vigorous physical activity in people with TBI.