The main purpose of this study was to examine pathways from maternal parenting behavior to externalizing problem behavior through deviant peer affiliation and self-concept. A sample of 405 high school students (184 boys and 221girls) in the Gyeonggi-do region of South Korea completed questionnaires focusing on maternal monitoring and adolescent deviant peer affiliation, self-concept, and externalizing problem behavior. Data were analyzed by structural equation modeling. The results demonstrated that maternal monitoring indirectly influences adolescent externalizing problem behavior through deviant peer affiliation and/or self-concept. When maternal monitoring was perceived as low by adolescents, they were more involved with deviant peers. This was followed by a more negative self-concept. Finally, negative self-concept resulted in more externalizing problem behavior by adolescents. The results emphasize that deviant peer affiliation plays a crucial role in increasing adolescent externalizing problem behavior.