Background: Many studies have recently reported that vitamin D deficiency may be associated with obesity. However, the relationship between vitamin D deficiency and obesity in Korean adolescents has yet to be sufficiently studied. Therefore, this study aims to reveal the relationship between vitamin D levels and obesity (or body mass index, BMI) in Korean adolescents. Methods: Among those who participated in the Korean National Health and Nutrition Examination Survey (2012–2014), we selected 1,238 adolescents aged 12 to 19 who underwent tests for determining 25-hydroxyvitamin D (25(OH)D) levels. The odds ratio of obesity according to 25(OH)D concentration was calculated using logistic regression analysis. The odds ratio of obesity in the vitamin D deficiency group compared with the vitamin D sufficiency group was calculated in the same way. The influence of 25(OH)D concentration on BMI was analyzed using regression analysis. All results were adjusted for variables, such as biologic sex, age, household income level, area of residence, physical activity, stress perception, sleeping hours, alcohol drinking, and smoking. Results: The mean 25(OH)D concentration of subjects was lower than the 20 ng/mL. In both male and female subjects, 25(OH)D concentration did not have any influence on obesity. Furthermore, no difference in obesity was found between the vitamin D deficiency and sufficiency groups. However, when 25(OH)D concentration was increased by 1 ng/mL, BMI was decreased by 0.043 kg/m 2 . Conclusion: In Korean adolescents, the vitamin D level increased as the BMI decreased.