Objective To investigate cognitive function in obese Egyptian adolescents. Methods A stratified-cluster proportionate random sampling was used to select a representative sample of 4899 pupils in preparatory school aged 12–15 year. Overweight was defined as BMI ≥85th but <95th percentile, while obesity as ≥95th BMI percentile, using Egyptian percentile. Wechsler Intelligence Scale and Executive Function test were used to assess cognitive function. Results The prevalence of overweight was 14.9% and obesity was 6.6%. The children’s mean full scale IQ was 89.54. Wechsler Intelligence Scale revealed significant differences in performance intelligence quotient (PIQ) scores between overweight and obese children. Parental educational was significantly related to total intelligence quotient (TIQ) (p 0.03). Executive function was significantly impaired in obese adolescents. Executive function was significantly lower in obese than overweight adolescents. After adjustment for age and parental education level female obese adolescents had lower cognitive function than male adolescents. BMI and waist circumference were the best predictor of impaired cognitive function in obese and overweight adolescents. Conclusion Overweight and obesity were associated with cognitive dysfunction in adolescents.