Purpose: This study was conducted to develop an expert competency model for preventing adolescent addictive behavior and to analyze educational needs of psychiatric mental health nurses in Korea. Methods: The process involved competency modeling using to the expert’s delphi construction and analyzing validity using to the development of the expert competency scale. Participants were 441 experts and 137 psychiatric mental health nurses in 2 cities. Results: The expert competency model for preventing adolescent addictive behavior included positive self-worth, capability of adolescents comprehension, environment comprehension capability of adolescent, capability of inner motivation, perception of personal competency, communication skills with adolescent, addiction comprehension capability, and addiction management skills. The developed expert competency scale demonstrates good reliability and validity. Psychiatric mental health nurses’ highest educational needs from expert competency scale were addiction management skills and communication skills with adolescents. Conclusion: The study findings indicate that the expert competency model for preventing adolescent addictive behavior has good validity and reliability when applied to adolescents. It provides the first step toward developing an education program or guidelines for experts including psychiatric mental health nurses.