Background: Mental illnesses are among the leading causes of disability worldwide. Individuals with severe mental illnesses were reported to have a significant reduction in life expectancy and mortality rate by two to three folds compared to the general population. However, adults with mental illnesses are at higher risk of developing metabolic changes such as obesity, hypertension, dyslipidemia and hyperglycemia. The inability or failure to diagnose metabolic syndrome among the mentally ill population promptly has dire consequences, which include the development of other co-morbidities and decreased life span with a substantial financial burden. Despite the importance of this topic, very few researches were conducted in the Arab Gulf Cooperation Council about the prevalence and associated factors of metabolic syndrome among mentally ill patients and none in Oman. Purpose: This study was conducted to assess the prevalence of metabolic syndrome among Omani adults with mental illnesses and explore the associated factors of the metabolic syndrome. Methods: A descriptive cross-sectional design was used to identify the prevalence and associated factors of metabolic syndrome among Omani adults with mental illnesses (n= 251) in two tertiary hospitals at the outpatient department at Sultan Qaboos University Hospital and Al Masarra- Hospital in Muscat, Sultanate of Oman using a convenience sampling technique. Data were collected using the World Health Organization Quality of Life questionnaire (WHOQOL-BREF) to assess the quality of life. The proportion of days covered (PDC) and the Medication Adherence Rating Scale (MARS) was also used to measure patients' medication adherence. Moreover, the Godin Shephard Leisure-Time Physical Activity Questionnaire (GSLTPAQ) measures the physical activity level of patients. Descriptive analysis including the mean, standard deviation, frequency and percentages used to describe the sample characteristics. Correlational analyses and binary logistic regression analyses were also conducted. Results: The overall prevalence of metabolic syndrome was found as follows, 29.9%, 30.3% and 33.1% based on Modified Adult Treatment Panel III-A (ATP III-A) criteria, International Diabetes Federation (IDF) criteria and the Joint statement of IDF Task Force with other organizations criteria respectively. The regression analysis showed that being on anti-diabetic medication, larger waist circumference size and higher triglyceride blood level positively predicted metabolic syndrome, while activity level negatively predicted the metabolic syndrome. Conclusion: Findings of this study can be utilized by policymakers and relevant authorities to develop policies and establish screening protocols for metabolic syndrome among individuals with mental illnesses. Additionally, these findings can contribute to developing screening and prevention strategies to mitigate the negative consequences of metabolic syndrome and improve the overall quality of life for this population.