To investigate the prevalence of obesity among elementary school children and to examine the association between obesity and caries activity in the mixed dentition stage. Methods: This cross-sectional study was conducted in King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia between September 2014 and June 2015 using a multi-stage stratified sample of 915 elementary school children (482 boys, 433 girls) in Jeddah, Saudi Arabia. Anthropometric measurements, consisting of height, weight, body mass index (BMI), and waist circumference (WC), were obtained. Children were classified as underweight/healthy, overweight, or obese and as non-obese or obese according to their BMI and WC, respectively. Each child's caries experience was assessed using the decay score in the primary and permanent teeth. Results: Based on BMI, 18% of children were obese, 18% were overweight, and 64% were underweight/normal. Based on WC, 16% of children were obese, and 84% were non-obese. Girls had a significantly higher prevalence of obesity based on WC measurements (p less than 0.001), but not BMI. Children enrolled in private schools had a significantly higher prevalence of obesity (p less than 0.05) than those in public schools. For primary and permanent teeth combined, children with higher BMI and WC had a lower prevalence of caries (p less than 0.05). Conclusion: The prevalence of obesity was high among male and female elementary school children. Overall caries activity was inversely proportional to BMI and WC.