Introduction: The rising rate of childhood obesity and the serious health problems it causes are gaining increasing attention in medical research and health policy. Aim: This study aimed to evaluate the relationship between insulin resistance and the oxidative stress biomarker 8-iso-prostaglandin F_2α levels in obese children. Materials and methods: Forty-four children in total (21 boys and 23 girls) aged between 6 and 15 years and diagnosed with obesity who attended the Pediatric Endocrinology Unit between December 2020 and June 21 were enrolled in our study. Forty children (20 boys and 20 girls) without systemic diseases were selected as controls. From the percentile curves determined for Turkish children, percentile values of obese children and control group were calculated based on sex and age. In addition, the insulin resistance values (HOMA-IR) in the homeostasis model were calculated. The relationship between the variables was examined with the Pearson and Spearman correlation tests. Children between the 5th and 85th percentile were defined as the control group, and those above the 95th percentile were defined as the obese group. Systolic and diastolic blood pressure, triglyceride, total cholesterol, LDL cholesterol, HDL cholesterol, fasting blood sugar (glucose), insulin, and 8-iso-PGF_2α concentrations were measured in all children included in the study. Results: There were significant differences between the two groups in terms of age, body mass index, and systolic and diastolic blood pressures (p<0.05). Glucose, triglyceride, insulin, 8-iso-PGF_2α and HOMA-IR levels were found to be statistically significantly higher in obese children than the levels in the control group (p<0.05). In addition, significant positive correlations were found between insulin levels and glucose, triglyceride and HOMA-IR values in obese patients (p<0.05). In obese children, 8-iso-PGF_2α concentrations were found to be statistically significantly higher than those in the control group (p<0.01). ROC analysis had a good diagnostic value for 8-iso-PGF_2α where the area under the curve was 1.0. A direct, positive, statistically significant correlation was found between insulin resistance and the 8-iso-PGF_2α values (r=0.420, p=0.037). Conclusions: 8-iso-PGF_2α concentrations were found to be higher in obese children than in the control group. It was observed that increased insulin resistance raised 8-iso-PGF_2α levels. 8-iso-PGF_2α is thought to be particularly important for the diagnosis and treatment of these patients, with 99% sensitivity and specificity.