A new sildenafil analogue was found to have been added illegally to a dietary supplement marketed for the enhancement of sexual function. The structure was determined as 5-(2-ethoxy-5-{[4-(3,5-dimethyl)piperazin-1-yl]-acetyl}phenyl)-1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo [4,3-d]pyrimidin-7-one. Owing to the inclusion of a dimethyl group in acetildenafil, the detected compound was called dimethylacetildenafil. The sample was purified with column chromatography. The IR, LC/MS (ESI), and completely assigned NMR data of dimethylacetildenafil are reported for the first time. The structure was compared with that of acetildenafil and the results showed that the ethyl substitution at the 4-position of the piperazine ring had been replaced by a 3,5-dimethyl substitution. This new acetildenafil analogue was isolated and identified for the first time.