The reported rates of HIV/AIDS cases and risky sexual behaviors have increased among adolescents in South Korea. This study reports the presence, impact, and development of optimistic bias toward HIV/AIDS in Korean adolescents. Survey data from 2996 participants were randomly collected from 8 high school and 8 middle school students in Seoul, South Korea. This study found that Korean high and middle school students had greater optimistic bias toward HIV/AIDS compared with their peers (P = .000). Involvement (r = -0.19; P < .01) decreased optimistic bias, whereas self-esteem (r = 0.13; P < .01) and self-control (r = 0.22; P < .01) increased it. Univariate analysis results indicated that gender and academic performance significantly influenced optimistic bias. However, according to hierarchical multiple regression, this significance was lost, and involvement, self-esteem, and self-control exerted a greater effect. Equipping Korean adolescents with involvement, awareness, and tailored interventions will reduce optimistic bias and yield greater protection against HIV/AIDS.