Physical activity (PA) in patients with traumatic brain injury (TBI) may be impaired leading to secondary health issues and limitations in participation.This study aims to determine the level of PA and its determinants in adolescents and young adults with TBI.Cross-sectional survey study.Outpatient clinic of a rehabilitation centre.Discharged patients aged 12-39 years with a diagnosis of TBI >6 months treated in the rehabilitation centre between 2009-2012.The Activity Questionnaire for Adults and Adolescents (AQuAA) measuring PA, with results dichotomized for meeting or not meeting Dutch recommendations for health enhancing physical activity (D-HEPA) and the Checklist Individual Strength (CIS; range 20- 140, higher scores represent higher levels of fatigue), measuring fatigue, were administered.Fifty (47%) of the 107 invited patients completed the questionnaire. Mean age was 25.0 years (SD 7.2)) and 22 (44%) were male. Eighteen (36%) had a mild injury, 13 (26%) a moderate injury and 19 (38%) a severe injury. Median time spent on moderate-vigorous physical activity was 518 minutes/week (IQR 236-1725) (males performing significantly more minutes on moderate-vigorous activity than women) and on sedentary activity 2728 minutes/week (IQR 1637-3994). Thirty-two (64%) participants met the D-HEPA. According to the CIS, 19 participants (38%) were severely fatigued. Both the CIS total score and the subscales motivation and physical activitywere associated with meeting the D-HEPA.The proportion of individuals with TBI meeting D-HEPA was similar to the general population, with the PA level being associated with self-reported fatigue.Physical activity programmes are continuously being developed to increase the percentage of individuals meeting public health recommendations for PA; when developing programmes for individuals with TBI extra consideration should be taken for the presence of fatigue. As in the general population, females with TBI are less active, PA programmes should probably consider gender differences in their development.