© TÜBİTAK.Background/aim: Thiol status is a good reflector of the cellular redox and have vital roles in various cellular signaling pathways. The purpose of the study was to investigate thiol status in patients with SARS-CoV-2 infection. Materials and methods: A total of 587 subjects (517 patients/70 healthy controls) were enrolled in the study.The patients were categorized into the groups regarding to the severity of disease (mild, moderate, severe, and critical).Thiol status of all groups were compared. Results: The patients had significantly diminished thiol levels compared to controls. Thiol levels were gradually decreased as the severity of the disease increased. Logistic regression analyses identified that thiol concentrations were an independent risk factor for the disease severity in each phase (mild group OR 0.975, 95%CI 0.965-0.986; moderate group, OR 0.964, 95%CI 0.953-0.976; severe group OR 0.953, 95%CI 0.941-0.965; critical group OR 0.947, 95%CI 0.935-0.960).Thiol test exhibited the largest area under the curve at 0.949, with the highest sensitivity (98.6%) and specificity (80.4%). Conclusions: Depleted thiol status was observed in SARS-CoV-2 infection. Decline of the thiol levels by degrees while the severity of infection increased was closely related to the progression of the disease. This outcome highlights that thiols could be an impressible biomarker for predicting of the severity of COVID-19.