Overweight and obesity are considered a serious health problem in Thailand. This study examined the prevalence of overweight and obesity in a nationally representative sample of Thai children and adults based on international standards. A cross-sectional population survey of 16,596 Thais aged 3 years and over was conducted. Heights and weights were obtained using standardized methods. Estimates of the overweight and obesity prevalence in children, adolescents, and adults were computed. The prevalence of overweight and obesity among children and adolescents aged 3 to 18 years was 7.6% and 9.0%, respectively, and was higher among boys than girls. Among adults, using the the Regional Office for the Western Pacific (WPRO) standard, 17.1% of adults were classified as overweight [body mass index (BMI) 23.0-24.9 kg/m²], 19.0% as class I obesity (BMI 25.0-29.9 kg/m²), and 4.8% as class II obesity (BMI ≥ 30.0 kg/m²). Using the World Health Organization (WHO) definition, 19.0% were overweight (BMI 25-29.9 kg/m²), 4.0% class I obesity (BMI 30.0-34.9 kg/m²), 0.8% class II obesity (BMI 35.0-39.9 kg/m²), and 0.1% class III obesity (BMI ≥ 40.0 kg/m²). There was a vast difference in obesity prevalence between the WHO and the WPRO criteria. Obesity prevalence when using the WPRO definition (23.8%) was almost five times greater than when defined with the WHO standard (4.9%). The present study found a high prevalence of overweight and obesity in nationally representative sample of the Thai population. Higher rates of overweight and obesity prevalence were computed using the WPRO standard when compared to the WHO standard.