The continuous emergence of severe acute respiratory coronavirus 2 (SARS-CoV-2) variants and the increasing number of breakthrough infection cases among vaccinated people support the urgent need for research and development of antiviral drugs. Viral entry is an intriguing target for antiviral drug development. We found that diltiazem, a blocker of the L-type calcium channel Cav1.2 pore-forming subunit (Cav1.2 α1c) and an FDA-approved drug, inhibits the binding and internalization of SARS-CoV-2, and decreases SARS-CoV-2 infection in cells and mouse lung. Cav1.2 α1c interacts with SARS-CoV-2 spike protein and ACE2, and affects the attachment and internalization of SARS-CoV-2. Our finding suggests that diltiazem has potential as a drug against SARS-CoV-2 infection and that Cav1.2 α1c is a promising target for antiviral drug development for COVID-19. Author summary: The emergence of variants of SARS-CoV-2 and the breakthrough infections that have occurred in recipients of approved SARS-CoV-2 vaccines raise doubts about the effectiveness of the vaccines and highlight the importance of antiviral drugs. An ideal drug to treat COVID-19 should be safe, affordable, and accessible. However, remdesivir remains the only authorized drug approved by the US FDA for emergency use, and it appears to have little effect on hospitalized COVID-19 patients. Therefore, identifying drugs to treat SARS-CoV-2 infections remains extremely important and urgent. In this study, we found that the calcium channel blocker diltiazem, which has been approved in the US since 1982 and is cheap and widely used in clinical practice for many indications, inhibits the binding and internalization of SARS-CoV-2, and decreases SARS-CoV-2 infection in cells and mouse lung. The L-type calcium channel Cav1.2 pore-forming subunit (Cav1.2 α1c), the main target of diltiazem, interacts and colocalizes with SARS-CoV-2 spike protein and ACE2, thereby affecting cell attachment and internalization of SARS-CoV-2. Our finding suggests that diltiazem could be candidate COVID-19 treatment and that Cav1.2 α1c may be a promising target for anti-SARS-CoV-2 drugs. [ABSTRACT FROM AUTHOR]