This study investigated the efficacy of a smartphone-based cognitive behavioral intervention for substance use among homeless youth at one shelter in Chicago. Homeless youth demonstrate high rates of psychiatric disorders, but there are significant barriers to mental health services. Previous studies indicate that mobile phones are easily accessible and frequently utilized by homeless youth, so mobile technology is a potential platform for providing mental health services to this population. The purpose of this study was to determine the efficacy of a smartphone-based cognitive behavioral intervention on substance use.