Effective treatments for?coronavirus?disease 2019 (COVID-19) are urgently needed to control this current pandemic, caused by?severe?acuterespiratory?syndrome?coronavirus?2?(SARS-CoV-2). Replication of SARS-CoV-2?depends on the viral?RNA-dependent?RNA?polymerase(RdRp), which is the likely target of the investigational nucleotide analogue?remdesivir?(RDV). RDV shows broad-spectrum?antiviral?activity against?RNA?viruses, and previous studies with RdRps from Ebola virus (EBOV) and Middle East?respiratory?syndrome?coronavirus?(MERS-CoV) have revealed that delayed chain-termination is RDV's plausible mechanism of action. Here, we expressed and purified active SARS-CoV-2?RdRp composed of the non-structural proteins nsp8 and nsp12. Enzyme kinetics indicated that this RdRp efficiently incorporates the active triphosphate form of RDV (RDV-TP) into?RNA. Incorporation of RDV-TP at position i caused termination of?RNA?synthesis at position i+3. We obtained almost identical results with SARS-CoV, MERS-CoV, and SARS-CoV-2?RdRps. A unique property of RDV-TP is its?highselectivity over incorporation of its natural nucleotide counterpart ATP. In this regard, the triphosphate forms of?2'-C-methylated compounds, including sofosbuvir, approved for the management of hepatitis C virus infection, and the broad-acting antivirals favipiravir and ribavirin, exhibited significant deficits. Furthermore, we provide evidence for the target specificity of RDV, as RDV-TP was less efficiently incorporated by the distantly related Lassa virus RdRp, and termination of?RNA?synthesis was not observed. These results collectively provide a unifying, refined mechanism of RDV-mediated?RNA?synthesis inhibition in coronaviruses and define this nucleotide analogue as a?direct-actingantiviral?(DAA).