Purpose/Objectives: This study was conducted to determine the association between undernutrition and dietary diversity among school-age children in southwestern Nigeria. Methods: A total of 600 school children were randomly selected from six private and six public schools in the region. A standardized FAO-published 24-hour diet recall questionnaire for calculating a dietary diversity score was adapted, tested, and used for all students. Weight and height measures were taken and weight-for-age, height-for-age, and weight-for-height Z scores were calculated to determine the prevalence of underweight, stunting, and wasting respectively. Results: The average age for students in the study was 8.3 years. Significant differences existed between students in private and public schools in BMI-for-age ("p" = 0.025), in dietary diversity scores ("p" = 0.034) and in undernutrition ("p" = 0.003). No private school children exhibited underweight or stunting, but 11.7% were overweight or obese. No public school students were overweight. Stunting ("p" = 0.024) and wasting ("p" = 0.018) correlated significantly with lower dietary diversity scores. The food groups at the lowest levels in the children's diet were organ meats, milk and milk products, eggs, and vitamin A rich fruits and vegetables. Applications to Child Nutrition Professionals: A dietary diversity score may be useful in identifying school children at risk of undernutrition and the food groups most often lacking in local diets. Methods of increasing dietary diversity in meals at school and at home are likely to benefit children at nutritional risk.