Vesicular stomatitis virus (VSV) represents a promising platform for developing oncolytic viruses, as well as vaccines against significant human pathogens. To safely control VSV infection in humans, small-molecule drugs that selectively inhibit VSV infection may be needed. Here, using a cell-based high-throughput screening assay followed by an in vitro transcription assay, compounds with a 7-hydroxy-6-methyl-3,4-dihydroquinolin-2(1H)-one structure and an aromatic group at position 4 (named vesiculopolins, VPIs) were identified as VSV RNA polymerase inhibitors. The most effective compound, VPI A, inhibited VSV-induced cytopathic effects and in vitro mRNA synthesis with micromolar to submicromolar 50% inhibitory concentrations. VPI A was found to inhibit terminal de novo initiation rather than elongation for leader RNA synthesis, but not mRNA capping, with the VSV L protein, suggesting that VPI A is targeted to the polymerase domain in the L protein. VPI A inhibited transcription of Chandipura virus, but not of human parainfluenza virus 3, suggesting that it specifically acts on vesiculoviral L proteins. These results suggest that VPIs may serve not only as molecular probes to elucidate the mechanisms of transcription of vesiculoviruses, but also as lead compounds to develop antiviral drugs against vesiculoviruses and other related rhabdoviruses.