A new tadalafil analogue was identified along with homotadalafil during routine screening of an adulterated dietary supplement using HPLC-DAD. The UV spectrum of this analogue was almost identical with that of tadalafil. This compound was isolated from the supplement by using semi-preparative HPLC and its structure was subsequently elucidated by performing Q-TOF/MS/MS and NMR spectroscopic experiments. The spectral data indicate that this tadalafil analogue is a dimeric compound that consists of an ethylamino group and two pretadalafil moieties. NOE experiments and comparison with 1H NMR spectra of tadalafil and trans-tadalafil suggested the trans-relationship between the substituents on piperidine rings in the pretadalafil moieties.