A new derivative of vardenafil was detected in an alleged herbal dietary supplement and identified as 2-(2-ethoxy-5-(4-(2-hydroxyethyl)piperazin-1-ylsulfonyl)phenyl)-5-methyl-7-propyl-imidazo[1,5-f][1,2,4]triazin-4(3H)-thione. Structure elucidation was carried out by LC–UV–MS/MS and NMR. Results obtained with high resolution MS and IR spectroscopy confirmed the proposed chemical structure. The compound was distinguished from hydroxyvardenafil, a second active substance identified in the same product, by the conversion of the oxo group to a thio group on the imidazo-triazin moiety. Hydroxythiovardenafil was therefore suggested as a proprietary name for the new molecule. This is the first paper to describe a thio-analog of vardenafil in a commercially available product.