question: Coronavirus disease 2019 (COVID-19) caused by coronavirus SARS-CoV-2 infection has now evolved into a worldwide crisis that triggers substantial morbidity and mortality. COVID-19 occurs more frequently and has more serious complications in patients with diabetes mellitus, but the underlying mechanisms remain largely elusive. Here, we summarize current and evolving concepts on the detrimental effect of hyperglycemia on SARS-CoV-2 infection and consequences, focusing on several key mechanisms underlying the link between diabetes and COVID-19. A better understanding of the mechanisms by which hyperglycemia worsens the prognosis of COVID-19 is critical for reducing the risk of SARS-CoV-2 infection and its associated mortality.