The Hepatitis C virus (HCV) is a major cause of liver failure and hepatocellular carcinoma, with about 170 million people infected worldwide. Replication of HCV in human cells requires the action of the HCV non-structural protein 3 (NS3), which exhibits both protease and helicase activities. Since there are numerous NS3 protease inhibitors but few NS3 helicase inhibitors, the goal here was to develop specific NS3 helicase inhibitors. During assay development, we discovered that trace components in the dye mixtures thioflavine S and primuline were potent inhibitors of the NS3 helicase. Based on the common structure of these components we designed ML283. The probe candidate is a potent inhibitor of the NS3 helicase enzyme and can be synthesized in the necessary quantities for further investigation. ML283 is more potent and specific than other recently reported NS3 helicase inhibitors under the same assay conditions, allowing a direct comparison. Preliminary experiments indicate that this compound is able to penetrate cells and inhibit HCV replication with no significant cytotoxicity. Thus the probe would be a useful tool for the investigation of viral helicases.