Background: Thymus has a pivotal role in combating infectious diseases. Although some reviews have been published about its critical role in COVID-19, there is not enough research. In this study, the size and density of thymus related to computed tomography pulmonary severity score (CT-SS) were researched.,Results: A total of 196 patients were analyzed with a mean age of 52.54 +/- 18.78 years; 97 (49.5%) of them were RT-PCR (-) and 99 (50.5%) were RT-PCR (+). Within RT-PCR (+) group 62 (62.6%) of them had pneumonia with a mean CT-SS of 9.37 +/- 8.83; within RT-PCR (-) group 20 (20.6%) of them had pneumonia with the mean CT-SS of 12.00 +/- 10.18. CT-SS had moderate negative correlation with thymus volume and thymus maximum diameter in patients having nodular-type thymus (R = -0.591, P = 0.02; R = -0.515, P = 0.049, respectively). Homogenous fat infiltration was more commonly seen in RT-PCR (-) group while reticular and nodular types were commonly seen in RT-PCR (+) group (p = 0.015). The mean volume and maximum diameter of thymus were statistically significantly higher in RT-PCR (+) group (p = 0.027 and p = 0.048, respectively).,Conclusion: This study showed the higher thymic volume and maximum diameter and more involution in COVID-19 patients. CT-SS had a moderate negative correlation with thymus volume and thymus maximum diameter. Pneumonia was more frequent in COVID patients, but mean CT-SS of the non-COVID cases was higher.