BACKGROUND: Adults who suffer from psychiatric disorders report low levels of physical activity and the activity levels differ between disorders. Less is known regarding physical activity across psychiatric disorders in adolescence. We investigate the frequency and type of physical activity in adolescent psychiatric patients, compared with adolescents in the general population. METHODS: A total of 566 adolescent psychiatric patients aged 13-18 years who participated in the CAP survey, Norway, were compared to 8173 adolescents aged 13-19 years who participated in the Nord-Trøndelag Health Study, Young-HUNT 3, Norway. All adolescents completed a questionnaire, including questions about physical activity and participation in team and individual sports. RESULTS: Approximately 50% of adolescents with psychiatric disorders and 25% of the population sample reported low levels of physical activity. Within the clinical sample, those with mood disorders (62%) and autism spectrum disorders (56%) were the most inactive and those with eating disorders (36%) the most active. This pattern was the same in individual and team sports. After multivariable adjustment, adolescents with a psychiatric disorder had a three-fold increased risk of lower levels of physical activity, and a corresponding risk of not participating in team and individual sports compared with adolescents in the general population. CONCLUSIONS: Levels of physical activity were low in adolescent psychiatric patients compared with the general population, yet activity levels differed considerably between various disorders. The findings underscore the importance of assessing physical activity in adolescents with psychiatric disorders and providing early intervention to promote mental as well as physical health in this early stage of life.