The main protease (Mpro) of SARS-CoV-2 is an attractive drug target for antivirals, as this enzyme plays a key role in virus replication. Drug repurposing is a promising option for the treatment of coronavirus disease 2019 (COVID-19). Recently, a number of FDA-approved drugs have been identified as Mpro inhibitors, but stringent hit validation is lacking. In this study, we rigorously reevaluated the in vitro inhibition of the Mpro enzyme by repurposed drugs via combined experiments, including the fluorescence resonance energy transfer (FRET) assay, fluorescence polarization (FP) assay, and protease biosensor cleavage assay. Our results from a set of in vitro assays revealed that boceprevir is a potential Mpro inhibitor, but other repurposed drugs, including atazanavir, dipyridamole, entrectinib, ethacridine, glecaprevir, hydroxychloroquine, ivermectin, meisoindigo, pelitinib, raloxifene, roxatidine acetate, saquinavir, teicoplanin, thonzonium bromide, and valacyclovir, are not. Our research highlights that the use of candidate Mpro inhibitors from primary screening warrants further comprehensive studies before the reporting of new findings.