Objective: Previous research has suggested that metabolic syndrome is more prevalent among patients with schizophrenia and bipolar disorder. Given the scarcity of comparative research on these disorders, this study aims to compare the prevalence of metabolic syndrome in two diagnostic groups and to identify sociodemographic or clinical factors related to metabolic syndrome. Methods: A total of 235 patients diagnosed with schizophrenia (n=160) and bipolar disorder (n=75) and were being followed up by a metabolic monitoring program in a specialized outpatient clinic and using antipsychotic therapy for at least eight weeks have participated in this cross-sectional study. Patients were diagnosed with metabolic syndrome according to National Cholesterol Education Program Adult Treatment Panel-III (NCEP ATP-III) criteria. Sociodemographic and clinical data including metabolic records were collected retrospectively by reviewing the patients' medical records. Results: The overall prevalence of metabolic syndrome was 32.3% and waist circumference was the most frequent individual component with a prevalence of 60%. There were no differences in the prevalence of metabolic syndrome between schizophrenia and bipolar patients. Age, body mass index (BMI), and use of certain types of atypical antipsychotics were found related to metabolic syndrome. Conclusion: The prevalence of metabolic syndrome and its components among patients with severe mental illness is alarming irrespective of their diagnoses. Thus, metabolic screening is crucial especially for high-risk groups.