Significance The SARS-CoV-2 spike protein is responsible for host receptor recognition, membrane fusion, and viral infection. Understanding the cellular and inhibiting the molecular mechanisms of spike-driven viral entry is a research priority in curbing the ongoing pandemic and preventing future coronavirus outbreaks. Here, we highlight that the generation of SARS-CoV-2 S2′ fragments, a proteolytic event occurring within the S2 subunit, is a molecular switch coupled to membrane fusion. Downstream of host receptor recognition, spike-driven syncytia formation requires the presence of an S2′ cleavage site at arginine 815 but not 685. Hence, the proteolytic processing of spike at the S2′ site upon its engagement of host ACE2 may serve as a potential antiviral target against the current SARS-CoV-2 and related coronavirus strains.