Unknown impurity associated with the degradation process of dapoxetine base was isolated. The structure elucidation of this new compound using accurate mass data, IR and NMR spectroscopy is presented herein. The unambiguous resonance assignment concluded to the formation of geometrical isomers of cinnamyloxynaphtalenes via Cope elimination of dapoxetin-N-oxide, the major oxidative and metabolic degradation product of dapoxetine. An efficient and simple synthetic approach has also been developed for the synthesis of dapoxetine-N-oxide for the first time and cinnamyloxynaphtalene in order to confirm the proposed degradation pathway and structures of the degradation products. It was observed that the main degradation product of dapoxetine base when exposed to air is 1-(2E)-cinnamyloxynaphthalene, while its Z isomer was also confirmed as a minor impurity.