To determine the prevalence of metabolic syndrome and examine its association with patients with major depressive disorder. Methods: All patients diagnosed with major depressive disorder (MDD) who visited the Psychiatric Outpatient Clinic at Maharaj Nakorn Chiang Mai Hospital were invited to participate in this study. Subjects who met the inclusion criteria and did not have exclusion criteria were included in this project until 140 subjects were enrolled. The criteria proposed by the American Heart Association/National Heart, Lung, and Blood Institute (updated ATPIII) were utilized for diagnosing metabolic syndrome. Age, history of antidepressant and antipsychotic use, time of illness, severity of illness and family history of metabolic syndrome were assessed as factors associated with metabolic syndrome. Data was analyzed by using percentage, mean, standard deviations, student t-test, chi-square test and Fisher’s exact test. Results: One hundred forty subjects were recruited. The prevalence of metabolic syndrome determined by the American Heart Association/National Heart, Lung, and Blood Institute (updated ATPIII) was 37.9% (53). Age was significantly associated with metabolic syndrome in patients with a major depressive disorder (p≤0.001). The duration of illness, duration of treatment, family history of diabetes, hypertension and dyslipidemia were not associated with metabolic syndrome. Conclusions: A high proportion of Thai patients with MDD had metabolic syndrome. These findings support the importance of assessing and monitoring metabolic syndrome in MDD patients, especially in older patients.