Diabetes, as the major cause of hyperglycemia, is a chronic metabolic disease. Immune system disorders caused by diabetes can increase the risk of respiratory diseases. Thus, diabetes is considered to be a major risk factor for viral respiratory infections such as coronavirus infections. Coronaviruses are members of the Coronaviridae, which has caused three outbreaks from 2003 to 2020. Patients with coronavirus infection in the lower and upper respiratory tract could show mild to severe symptoms. In this review, we focus on the relationship between diabetes and three coronaviruses: SARS-CoV, MERS-CoV, and SARS-CoV-2.