Background. Psychiatric illnesses involve alterations in the brain or nervous system function and result in altered perception, responses to the environment, and daily functioning, which ultimately diminish the capacity to cope with the regular demands of life, including nutritional intake. Little is known about nutritional status in psychiatric patients in our setup, particularly in northeast Ethiopia. Thus, this study is aimed at assessing the nutritional status and associated factors among adult psychiatric patients in Dessie referral hospital, northeast Ethiopia. Methods. A facility-based cross-sectional study was employed among 530 psychiatric patients using an interviewer-administered structured questionnaire, and anthropometric measurements were taken from April 10 to June 20, 2018. Multinomial logistic regression analysis was used to identify the associated factors with the nutritional status of adult psychiatric patients. Results. The study revealed that the proportion of undernutrition and overnutrition was 20.0% (95% CI: 16.80-23.60) and 23.4% (95% CI: 19.80-27.00), respectively. Being male (AOR: 2.39, 95% CI: 1.28-4.47), private employed (AOR: 0.08, 95% CI: 0.02-0.31), and not consuming alcohol (AOR: 0.20, 95% CI: 0.56-0.74) were factors associated with undernutrition. Whereas not involved in physical activity (AOR: 2.98, 95% CI: 1.37-6.49), being newly diagnosed patient (AOR: 1.86, 95% CI: 1.01-3.42), and not chewing Khat (AOR: 3.92, 95% CI: 1.63-9.42) were factors associated with overnutrition of adult psychiatric patients. Conclusion. The proportion of both undernutrition and overnutrition was above the national average. Notably, nutrition significantly affects the production and use of neurotransmitters, and this may result in significant effects on physical, mental, or emotional processes. This undoubtedly affects the stabilization processes of neuropsychiatric patients. Therefore, healthcare managers, supervisors, and policymakers identify those vulnerable groups early and thereby design effective nutritional strategies to intervene in malnutrition among adult patients with a psychiatric disorder.