Treatment motivation is one of the basic requirements for successful treatment and rehabilitation in youth detention. Treatment motivation is often presumed, but rarely assessed, because there is a lack of reliable and valid measurement instruments to investigate treatment motivation in detained youth, especially in Germany. The Adolescent Treatment Motivation Questionnaire (ATMQ) provides insight into the treatment motivation of young prisoners. The present study examined the construct validity and reliability of the German ATMQ in a sample of 76 German delinquents. A confirmatory factor analysis showed a good fit to the data, indicating construct validity. Cronbach’s alpha reliability coefficients were good. Concurrent validity was supported by significant relations between treatment motivation and living group climate. The German ATMQ (8 items) can be used to assess treatment motivation in judicial and forensic psychiatric care in order to improve treatment motivation.