Purpose: The purpose of study was to examine the relationship between obesity and its associated factors (psychiatric symptom, duration of illness, type of medication, physical activity, dietary habits, depressive symptom, and stress) in patients with chronic mental illness registered to community mental health welfare centers. Methods: This was a cross-sectional correlation study using a convenience sampling. A total of 392 participants were recruited from community mental health welfare centers. The obtained data were analyzed using binary and multinomial logistic regression. Results: Atypical antipsychotic medication, duration of illness, dietary habits (overeating, and drinking instant coffee) were significantly contributed variables into body mass index (BMI) obesity. Atypical antipsychotic medication and instant coffee were significantly related to abdominal obesity. Conclusion: These results emphasized the needs of tailored obesity-preventive management for the community-dwelling patients with chronic mental illness, topically focusing on the administration of atypical antipsychotic medication, duration of illness, and dietary habits.