COVID-19's infection rate among adolescents is increasing; hence, it is important to prevent it as it can spread in the community through transmission in schools. It is crucial to determine the extent to which adolescents follow COVID-19 preventive measures and identify factors relating to such behaviors to implement more effective health education. This study aimed to understand factors related to COVID-19 preventive behaviors using the theory of planned behavior (TPB) and the health belief model (HBM) among adolescents in South Korea. Design and methods: A cross-sectional face-to-face questionnaire survey was conducted among 272 adolescents in South Korea. Structural equation modeling was used to examine the relationships between perceived susceptibility, perceived severity, attitude, subjective norms, perceived behavioral control, intention, and COVID-19 preventive behaviors. Results: Among adolescents' COVID-19 preventive behavior scores, wearing masks was the highest and distancing was the lowest. Adolescents' adherence to COVID-19 preventive behaviors was directly or indirectly associated with their perceived susceptibility, perceived severity, subjective norms, perceived behavioral control, and intention. Conclusions: Attention to mitigate social isolation is important to improve compliance with COVID-19 preventive behaviors. Perceived susceptibility, perceived severity, subjective norms, perceived behavioral control, and intention explained 61.3% of adolescents' COVID-19 preventive behaviors, there is a need for education to promote relevant factors. Practice implications: To increase adolescent awareness of COVID-19 and increase attitudes, subjective norms, and perceived behavior control, nurses need to provide education including the evidence for COVID-19 preventive behavior to increase adolescents' understanding and active implementation of those behaviors.