Coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has posed a remarkable global challenge as a previously unrecognized viral infection with high infectivity. The high prevalence of diabetes makes it one of the most common comorbidities observed in patients with COVID-19 infection. Studies have suggested that diabetes is associated with unfavorable COVID-19-related outcomes. Lower mortality rates have been observed among individuals with good glycemic control than among those with poor glycemic control, and glycemic management may affect the prognoses and outcomes of COVID-19 infection. Several studies have demonstrated that COVID-19 infection contributes to hyperglycemia, metabolic dysfunction, and increased microvascular complications and thrombotic events against the backdrop of aberrant endothelial function. Metformin, the most prescribed oral antidiabetic medication, exhibits multiple beneficial effects beyond its glucose-lowering functions, such as anti-infection, anti-inflammation, immunomodulation, anti-hypertension, preventive effects on chronic obstructive pulmonary disease and heart failure. Hence, metformin may share similar mechanisms of these pleiotropic benefits or contribute to the reduced morbidity/mortality in COVID-19 infection. Therefore, metformin is a feasible candidate drug for repurposing to address the rising number of patients with COVID-19 infection who have diabetes. This review article presents a summary of the multiple potential mechanisms through which metformin produces beneficial therapeutic effects and summarizes real-world data supporting the repurposing of metformin for use in patients with COVID-19 who have type 2 diabetes.