Background: We explored the relationship between body mass index-for-age percentile, body image distortion, and unnecessary weight loss efforts in Korean adolescent girls who are underweight and normal weight and examined the mediating effect of body image distortion on weight loss efforts. Methods: This study used data from the 2013 Korea Youth Risk Behavior Web-based Survey conducted by the Korea Centers for Disease Control and Prevention. The study sample comprised 29,633 Korean adolescent girls who are normal weight and underweight. Chi-square tests and correlations were used to assess bivariate relationships between the variables. To assess the mediating effect of body image distortion on the relationship between body mass index-for-age percentile and weight loss efforts, a 3-step regression analysis was used. Results: There was a significant correlation between body mass index-for-age percentiles and body image distortion, and both significantly correlated with weight loss efforts. In addition, a distorted perception of being overweight or obese was a significant mediating variable. Conclusions: School-based health education and intervention programs are needed to promote the formation of a positive body image, not only for obese adolescents, but also for normal-weight and underweight adolescent girls.