Objective: Explore the association between physically active behavior and obesity in 7-to 12-years-old Thai children. Material and Method: As part of SEANUTS Thailand, information on anthropometry, physical activity, and sociodemographic variables were collected in 7-to 12-years-old urban and rural Thai children. Multi-stage sampling technique was used and 1,345 children (32% urban, and 50.3% boys) participated in the study. Anthropometric measurements included weight, height, and BMI-for-age Z-scores (BAZ) using World Health Organization Growth Reference. Obesity was defined as BAZ >2SD. Physical activity was assessed using a validated physical activity questionnaire (PAQ). The PAQ provided an activity score, activity time in school, sleeping hours, and TV watching time as categorical variable, low, moderate, and high. Chi-square by likelihood ratio test and logistic regression were used to compare obese and non-obese groups. Results: The overall prevalence of overweight and obesity was 10.2 and 10.8% respectively, whereas 8.2% was classified as thin. Maternal education and religion did not differ between obese and non-obese children. However, obese children’s family income was higher. After controlling for family income, maternal education, and religion, obese children were significantly less active during break times in school, slept less, and watched more TV than non-obese. However, there was no difference in the activity score of obese and non-obese children. Conclusion: The study showed that physical activity during break time in school, sleep duration, and hours of TV viewing were associated with obesity in pre-adolescent Thai children. It is important to note that activity score was not associated with obesity. One of the most important benefits to be physically active in childhood is the potential to maintain this behavior into adulthood. Therefore, programs that encourage healthy behaviors and address these modifiable risk factors should be incorporated in the school curriculum.