Psychiatric and substance use disorders, separately or combined, are frequent in adolescence. Brief, psychometrically sound screening tools are needed to identify those at-risk. The Global Appraisal of Individual Needs-Short Screener (GAIN-SS) is a brief, inexpensive screener that was derived from the well researched full GAIN interview. However, its validity has not been independently investigated based on independent validated screening instruments in an adolescent clinical population. In this project, the internal consistency, validity, and optimal screening cutoff scores were examined in 95 adolescents, most of whom were receiving treatment, and suffered from internalizing (52%; n = 49), externalizing (66%, n = 63), and co-occurring substance use disorders (55%, n = 56). Results indicated adequate internal consistency and overall and subscale construct validity. Receiver operating characteristics (ROC) analyses revealed that the GSS substance use disorder subscale had adequate sensitivity (88%) and specificity (89%). Psychiatric disorder subscales performed less well. The discussion focuses on the strengths and potential weaknesses of use of the GSS and its potential utility in other populations.