Objectives: To determine self-reported physical activity (PA) levels and relationships with fatigue and sleep quality in adolescents and young adults after mild traumatic brain injury (mTBI). Setting: Follow-up 6–18 months after visiting the emergency department of one of 2 general hospitals. Participants: Forty-nine adolescents and young adults aged 12–25 years (mean 18.4 years), 22 (45%) male with mTBI. Design: Cross-sectional survey study. Main outcome measures: The Activity Questionnaire for Adults and Adolescents (AQuAA), with results dichotomized into meeting or not meeting Dutch Health Enhancing PA recommendations (D-HEPA), the Checklist Individual Strength (CIS, 4 subscores) and the Pittsburgh Sleep Quality Index (PSQI, total score) were administered. Results: Twenty-five participants (51%) did not meet the D-HEPA recommendations. After adjusting for sex, BMI and age, not meeting the recommendations was associated with a higher CIS Total Score (OR 1.04 95%CI 1.01, 1.07) but not with PSQI Total Score (OR 0.99, 95%CI 0.80, 1.21). Conclusions: In adolescents and young adults with mTBI the level of reported PA is associated with fatigue but not with sleep quality. It remains to be established whether interventions aiming to promote PA should primarily be focused on PA or fatigue or both.