Although the rate of sexual intercourse among adolescents has increased in Asian countries, including Korea, many sexually active adolescents still do not use contraception. The aim of this study was to identify the risk factors for contraceptive nonuse among adolescents using decision tree analysis of the 2018 Korea Youth Risk Behavior Survey data from 2,460 high school students who had an experience of sexual intercourse. The findings indicated that the highest risk group who did not use contraception during sexual intercourse did not receive sexual health education in school and was involved in habitual or purposeful drug use. The experience of ever receiving treatment due to violence and the experience of sexual intercourse after drinking were also identified as risk factors for contraceptive nonuse. To encourage contraceptive use, development of standard sexual health education, counseling, and educational intervention intended to prevent risky behaviors is needed.