The dotted white geometrid moth, Naxa seriaria Motschulsky (Lepidoptera: Geometridae), is a pest of Oleaceae in Korea, Japan, and China. In this study, we identified (3Z,6Z,9Z,12Z,15Z)-heneicosapentaene (C-21 pentaene) as the only compound in extracts of the pheromone glands of female N. seriaria causing a response from receptors on the antennae of males in analyses by gas chromatography with electroantennographic detection. The synthetic sex pheromone elicited dose-dependent electrophysiological responses from antennae of male N. seriaria. In field tests, more male moths were captured in traps baited with synthetic C-21 pentaene than in unbaited traps, and increasing the loading of C-21 pentaene in the lure increased catches of male moths. Significantly more male N. seriaria moths were caught in delta traps than in bucket traps. Based on these results, C-21 pentaene is proposed to be the major, if not the only, component of the sex pheromone of N. seriaria.