Objective: This study investigates the psychological impacts and their associated factors on patients with COVID-19 at a Thai field hospital. Methods: All eligible patients confirmed to have COVID-19 at Thammasat University field hospital completed an online self-reported mental health screening questionnaire which collected sociodemographic data, their clinical characteristics, and used the depression, anxiety, and stress scale (DASS-21). Results: A total of 40 patients participated in the study. The depression rate was found to be 22.5%, while the anxiety rate was 30%, and the stress rate was 20%. Having a history of psychiatric disorder alone was significantly associated with a higher DASS-21 score (p = 0.001). Meanwhile, gender, age, level of education, occupation, living status, severity of COVID-19, and the number of days admitted to hospital prior to the field hospital were not found to be associated with the DASS-21 scores (P > 0.05). Conclusion: The depression, anxiety, and stress symptoms in patients with COVID-19 at the field hospital were common. Patients with a history of psychiatric disorder should undergo specific evaluation during the isolation phase.