The prevalence of obesity among children and adolescents has been rapidly increasing in recent years. Obese individuals are at risk for vitamin D deficiency. The aim of this study was to investigate the relation of vitamin D deficiency with puberty and insulin resistance in obese children and adolescents. A total of 106 children and adolescents (48 prepubertal and 58 pubertal) between 8 and 16 years of age were included in the study. Fasting blood glucose, insulin, lipid profile, calcium, phosphorus, alkaline phosphatase, parathyroid hormone, 25-hydroxyvitamin D [25(OH) D] levels, as well as blood glucose and insulin concentrations at 120 min of oral glucose tolerance test were measured. Insulin resistance was calculated using the homeostasis model assessment. Daily vitamin D intake was questioned. Serum 25(OH) D level was normal in only 3.8%, insufficient in 34.0%, and deficient in 62.2% of the subjects. There was a statistically significant rate of 25(OH) D deficiency in the pubertal group compared with that in the prepubertal group. Those subjects with 25(OH) D deficiency were found to have greater insulin resistance. Vitamin D deficiency is common among obese children and adolescents. Low vitamin D levels in obese individuals may accelerate the development of metabolic syndrome, type 2 diabetes mellitus, and cardiovascular disease by further increasing insulin resistance.