This study determines the prevalence of metabolic syndrome (MetS) according to the International Diabetes Federation (IDF) and National Cholesterol Education Program III (NCEP) criteria in Thai adults. Data from a national representative sample, InterASIA study, including a total of 5305 Thai adults 35 years and older were analyzed. Overall, the age-standardized prevalence of MetS by IDF and NCEP criteria were 24.0% (men 16.4%, women 31.6%) and 32.6% (men 28.7%, women 36.4%), respectively. The difference in prevalence of MetS between genders was much greater for the IDF compared with the NCEP definition. The age-standardized prevalence rates distributed by geographic region were relatively uniform with a lowest prevalence in the northeast. Among all possible sets of components for MetS, the most common combinations were a set of low high-density lipoprotein cholesterol, high triglyceride, and hyperglycemia in men (3.9%) and a set of abdominal obesity, low high-density lipoprotein cholesterol, and high triglycerides in women (6.7%). MetS is common in Thai adults and NCEP definition captures more cases of MetS compared with the IDF definition. Implementation of programs to prevent obesity and metabolic factors along with future periodic survey to monitor the problem is crucial.