Objectives: Severe disease characterized by interstitial pneumonia may develop in some cases of coronavirus disease (COVID-19). Periostin has been associated with many respiratory diseases. In this study, we aimed to investigate whether periostin could be a useful new biomarker in the follow-up and severity assessment of the disease in patients with COVID-19 pneumonia. Methods: In the study, 32 patients followed up during May–July 2020 due to COVID-19 and 24 healthy controls were included. The patients were divided into two groups, namely, mild/moderate and severe, according to the severity of the disease. Serum periostin and transforming growth factor beta (TGF-β) levels were tested using an enzyme-linked immunosorbent assay (ELISA) method using commercially available ELISA kits. Results: It was observed that the periostin level was significantly higher in both mild/moderate cases and severe cases compared to the control group at first presentation. However, TGF-β levels at first presentation were similar between the groups. Conclusions: Our study is the first study to investigate periostin levels in patients with COVID-19, and we believe that periostin can be used as a new biomarker. Keywords: COVID-19, Periostin, TGF-β, Pneumonia, New Biomarker Coronaviruses are among the main pathogens that mainly target the human respiratory system. Severe disease characterized by interstitial pneumonia develops in 10-20% of patients. Periostin has recently been shown to be an indicator of disease progression in idiopathic pulmonary fibrosis and asthma. In this study, we aimed to investigate whether periostin could be a useful new biomarker in the follow-up and severity assessment of the disease in patients with COVID-19 pneumonia. This article demonstrated that periostin is a useful new biomarker for disease follow-up and severity in patients with COVID-19 pneumonia. It is also the first study on periostin levels in patients with COVID-19